Better MPG?

[jude_23_stark]

I have a 97 Jeep XJ with a 3'' lift and 235 75/15 tires. I drive it mostly on the highway. Is there anything I can do to impove my MPG? And what will give me the most bang for my buck? I have thought about an electric fan and different kinds of chips. What chip is better?

[Beej]

My experience says to slow down when driving, and accelerate smoothly when taking off. Cold air intake seemed to help mine, and so did replacing the sensors...

[jude_23_stark]

Which sensors? and how many MPG did you gain?

[j99xj]

My experience says to slow down when driving, and accelerate smoothly when taking off.

100% correct. The best way to gain mpg is to lighten you right foot.

Also nearly all the bolt on mods Dr. Dyno talks about on his site will increase mpg. However the most effective I believe is to jettison the stock belt driven fan for an electric fan (1.5 mpg). This mod is followed closely by the throttle body spacer (0.8 mpg) and the removal of the intake tube silencer. (0.75)

Thats about a 3 mpg increase. These mods will add 60 miles to the range of your tank. (assuming 20 gallon tank, which I think for the most part we all have)

Not bad at all I must say.

[MoabXJeeper]

Just a thought but you could try dropping your front drive line. I have been running without mine and doing about 5 mpg better. This is a 25% increase and I went from 280 miles per tank to about 320.

[Beej]

Which sensors? and how many MPG did you gain? All of them, just to make sure none are faulty as they are necessary for your engine to run efficiently. They're cheap anyway...

[jeepdude10000]

u did not say what MPG you are getting.

[Beej]

About 18 in the city and 20-22 highway. I don't credit the sensors, I credit the light foot...

[MaXJohnson]

100% correct. The best way to gain mpg is to lighten you right foot.

Also nearly all the bolt on mods Dr. Dyno talks about on his site will increase mpg. However the most effective I believe is to jettison the stock belt driven fan for an electric fan (1.5 mpg). This mod is followed closely by the throttle body spacer (0.8 mpg) and the removal of the intake tube silencer. (0.75)

Thats about a 3 mpg increase. These mods will add 60 miles to the range of your tank. (assuming 20 gallon tank, which I think for the most part we all have)

Not bad at all I must say.
BS

[MaXJohnson]

Just a thought but you could try dropping your front drive line. I have been running without mine and doing about 5 mpg better. This is a 25% increase and I went from 280 miles per tank to about 320.
an increase in your range from 280 miles to 320 miles amounts to a 14% increase, not 25%, but none the less, expecting a 14% (or 25%) increase in fuel mileage from a minor reduction in mechanical friction is foolish. A 10% reduction in rolling resistance is generally considered to reduce fuel consumption by about 2%

[Gnat5680]

I have a 97 Jeep XJ with a 3'' lift and 235 75/15 tires. Is there anything I can do to impove my MPG?

As said in another MPG topic a few month back.

1. Dump you 500lbs girlfriend
2. Get an 82 Yugo
3. put the original tires back on

other wise if you like the jeep i would go with the TB, electric fan, and the air tube. when i have time i will start on that one.

[jeepdude10000]

well my 89 XJ get between 18(city) - 24 MPG(highway)
my 89 Caddy gets 25(city) - 34 MPG(highway), and i still drive my XJ more, (the seats are nicer, way better stereo(new)).

[seanof30306@yahoo.]

100% correct. The best way to gain mpg is to lighten you right foot.

Also nearly all the bolt on mods Dr. Dyno talks about on his site will increase mpg. However the most effective I believe is to jettison the stock belt driven fan for an electric fan (1.5 mpg). This mod is followed closely by the throttle body spacer (0.8 mpg) and the removal of the intake tube silencer. (0.75)

Thats about a 3 mpg increase. These mods will add 60 miles to the range of your tank. (assuming 20 gallon tank, which I think for the most part we all have)

Not bad at all I must say.

I agree with lightening the foot, but other mods just for the sake of improving mileage can quickly become penny-wise and pound-foolish. My XJ gets 17mpg on the highway. I drive around 8,000 miles per year, so I use about 471 gallons of gas per year. My estimated fuel cost (assuming a price of 3.00 per gallon, for simplicity's sake), is 1410.00.

Let's say you can make those mods and increase your gas mileage by 3 mpg. (I've done a throttle body spacer and removed the intake tube silencer, by the way, and saw no gain in gas mileage). At 20 mpg and 8,000 miles per year, I would use about 400 gallons per year and my estimated fuel cost would be 1200.00. That's only a savings of 210.00.

Now, if I drove more miles, I'd save more money, but not that much more (unless I drove A LOT more miles). At 10,000 miles per year, for example, I'd spend 1765.00 on gas at 17mpg, and 1500.00 at 20mpg; a 265.00 yearly savings. At 12,000 miles per year, I'd spend 2118.00 on gas at 17mpg, and 1800.00 at 20mpg; a savings of 318.00.

I think it's pretty important to sit down and calculate the potential savings on gas from mods you're thinking of and compare that to the cost of actually doing them. Everyone's so crazy over gas prices these days that many go way overboard trying to improve their mileage.

[j99xj]

At 10,000 miles per year, for example, I'd spend 1765.00 on gas at 17mpg, and 1500.00 at 20mpg; a 265.00 yearly savings.

I drive about 10,000 miles a year.

Saving $265.00 a year on gas doesn't sound bad at all to me. That will easily pay for an electric fan, throttle body spacer, and the time to take out the silencer from the airbox.


Let's leave the Jeep spectrum and look at the the best mileage cars for a few moments. I want to explain TRUE FOOLISHNESS when it comes to saving money on cars/gas. This is what the car companies DO NOT want you and I to know.

Here is my comparison: A 2006 Honda Civic Hybrid, and a 2006 Honda Civic, they cost $22,150, and $14,760 respectively, and get 49 mpg, and 30 mpg city respectively.

As stated, I drive 10,000 miles a year. So the annual fuel costs would be $612 for the hybrid, and $1000 for the regular Civic, assuming $3.00 per gallon price. So I'm saving $388 a year with the hybrid......sounds good.

BUT WAIT A MINUTE! The hybird costs $7390 more up front! If I save $388 a year on gas, it would take me 19 YEARS to start saving money from the hybrid. Not to mention the poor little car would have a whopping 190,000 miles on it. (Which I doubt the hybrid was designed for, you would still be pushing it even with the regular Civic with that kind of miles on the car).

Unless the prices for hybrid comes down significantly, they will NOT save you a dime until the year 2025. Everytime I see a hybrid on the road I think to myself "What the hell were they thinking!?"

[seanof30306@yahoo.]

I drive about 10,000 miles a year.

Saving $265.00 a year on gas doesn't sound bad at all to me. That will easily pay for an electric fan, throttle body spacer, and the time to take out the silencer from the airbox.

Ok, I agree, IF those mods lead to those savings. As I said, I've done the throttle body spacer and removed the silencer and saw no improvement. Now, I only calculate my mileage in my head, and I round it off to the nearest even number, but I've had my cherokee for 10 years and know how many miles I get out of a tank of gas. I'm comfortable in saying there was no appreciable change; certainly not the 1.55 mpg j99xj says those two mods are worth.

Also, I'm having a hard time believing the belt-driven fan is costing 1.5 mpg. I haven't changed the one out on my Jeep, but I have on my 70 Chevelle with a 400 SBC. Not long after building the engine, I swapped the stock fan out for a fiberglass one. I saw no improvement in mileage or power, but at least it was less noisy. A few years later, I went with an aluminum radiator and put in dual electric fans while I was doing it. Again, very little change; MAYBE a .5 mpg increase, but then the question arises as to whether that was caused by the electric fan, or the cooler operating temperature from the improved cooling system as a whole.

Now, the fan on my Cherokee is half again smaller than the fan on my 400 SBC, I'm having a real hard time believing it's costing us 1.5 mpg. The factory was already installing one electric fan in later-model Cherokees (like mine), I just can't imagine their not going to dual electric fans to gain 1.5 mpg. Gas mileage was always a problem with Cherokees and SUVs in general, I believe they would have made that change in an instant to unlock that 1.5 mpg selling point.

Let's leave the Jeep spectrum and look at the the best mileage cars for a few moments. I want to explain TRUE FOOLISHNESS when it comes to saving money on cars/gas. This is what the car companies DO NOT want you and I to know.

Here is my comparison: A 2006 Honda Civic Hybrid, and a 2006 Honda Civic, they cost $22,150, and $14,760 respectively, and get 49 mpg, and 30 mpg city respectively.

As stated, I drive 10,000 miles a year. So the annual fuel costs would be $612 for the hybrid, and $1000 for the regular Civic, assuming $3.00 per gallon price. So I'm saving $388 a year with the hybrid......sounds good.

BUT WAIT A MINUTE! The hybird costs $7390 more up front! If I save $388 a year on gas, it would take me 19 YEARS to start saving money from the hybrid. Not to mention the poor little car would have a whopping 190,000 miles on it. (Which I doubt the hybrid was designed for, you would still be pushing it even with the regular Civic with that kind of miles on the car).

Unless the prices for hybrid comes down significantly, they will NOT save you a dime until the year 2025. Everytime I see a hybrid on the road I think to myself "What the hell were they thinking!?"

I totally agree. My best friend moved out into the country a few years ago, and now constantly complains about the cost of the commute in his Expedition. Like the flood of other people over the past year or so, he was ready to dump his Expedition for a smaller, more fuel-efficient car. The problem is, big SUVs are trading at an all-time low, and the stampede of people rushing to buy high mpg cars means there are no deals available.

I sat him down and made him do the math with me. When we did so, we found it would be 13 years before he saw any savings. I managed to talk him into waiting a bit until the manufacturers hit the market with a greater number of fuel-efficient cars and competition would force the prices down, and the current hysteria died down a bit and his trade-in value rebounded.

The problem isn't just with new cars, though. A woman I work with called her "mechanic" and asked how to get better mileage. He suggested a tune up, which cost her 240.00. There was nothing wrong with the way the car was running, and it only has 35,000 miles on it. She was at work the other night saying how thrilled she was to have a 2 mpg improvement. I didn't say anything, but I did a little math in my head. She bought the car in 2001, so she's putting about 7,000 miles per year on the car. Even at 3.00 per gallon, she's only saving 105.00 per year. That means it'll be more than two years before she breaks even, but there are more things to consider.

First, I question whether a car that is designed to go 75,000 miles between tune ups actually needed one at 35,000 miles.

Second, I'm not convinced she's actually seeing a 2 mpg improvement. People see what they want to see. She'd told people at work what she was going to do before she did it, and several of us pointed out that she shouldn't need a tune-up till 75,000 miles, etc. She would've looked pretty stupid if she'd come back in and said no improvement.

Third, who's to say any improvement she actually did see didn't come from having the tires properly inflated or the dirty air filter replaced (she'd only changed the oil "two or three times" since she bought the car, and I don't see her as someone who regularly checks her tire pressure).

I have used my Cherokee exclusively on the road since I bought it; I always knew I would. I waited till the end of the model year to buy to save money, but my selection was limited. I ended up finding one with everything I wanted (except power mirrors), but, it also had the Up-Country package, which I knew I'd never need. I'd negotiated a good enough deal that the cost of the package wasn't relevant, but it has either 3.73 or 3.55 gears, and I would have preferred 3.07.

Now, the rearend has been howling since I bought it, and there's been a "slipping" sound when I turn a corner after driving on the highway for any length of time for the past 25,000 miles or so (I think it's the clutches in the posi unit slipping). Both have been getting steadily worse, and it's time to get the rearend looked at. I've considered changing over to 3.07 gears when I do, but, since I'd have to change the gears in both differentials, the cost would wipe out any potential savings.

By the way, does anyone know how I find out what rear gear ratio I have? I found a VIN decoder, but it doesn't show ratios. Is there a build sheet in Jeeps? Also, does my Cherokee have a 7.5" or an 8.5" differential?

[jeepdude10000]

there should be a rag on ur diff cover,
http://www.coloradok5.com/axleguide.shtml

[seanof30306@yahoo.]

there should be a rag on ur diff cover,
http://www.coloradok5.com/axleguide.shtml

Looking at that chart, it looks like I have a Dana 35C

There's a white tag stuck on the differential with a bar code and the following:

PN 5730-5
8775300038

I looked under the back seat, but saw no build sheet.

[Beej]

Later model XJ's with auto trannies came with 3.55's and manuals came with 3.07's. 4 Cylinder models generally came with 4.10s. Even with stock tires, Cherokees are often considered to be geared too tall from the factory. Not just for offroading, but also for onroad pickmeup.

For ID ing the axle, there are a few ways. The Chrysler 8.25" has an obvious flat lip along on the bottom of the diff housing. The D35 has a round bottom when looked at straight on. The 8.25 axletubes are 3" in diameter while the D35's are 2.67". Hope this helps.

[j99xj]

Also, I'm having a hard time believing the belt-driven fan is costing 1.5 mpg.



Any belt driven accessory costs horsepower and mileage, every last one of them. Water pump, alternator, power steering, fan, and certainly the a/c (when on). They all have mass that must be accelerated along with the rest of the drivetrain when the rpms pick up; not to mention the energy they suck up to do the work they were designed to do. Reducing this mass, and what is called the "moment of interia" in physics will save energy for moving the car instead of the accessory. Thus the mileage increase.


The only accessory that is practical to get rid of on a street driven Jeep is the belt fan. I just leave my a/c off 75% of the time because I know what a drain it is. I actually turn my a/c ON when going down steep hills on the trail and highway because it helps with engine braking quite a bit.

[steelmen]

Am i missing something? Why cant i find throttle body spacers for 90 and earlier?

[Krazors001]

Am i missing something? Why cant i find throttle body spacers for 90 and earlier?
https://rustysoffroad.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=rustys&Category_Code=PER_AIR the tbs25.

[MaXJohnson]

Any belt driven accessory costs horsepower and mileage, every last one of them. Water pump, alternator, power steering, fan, and certainly the a/c (when on). They all have mass that must be accelerated along with the rest of the drivetrain when the rpms pick up; not to mention the energy they suck up to do the work they were designed to do. Reducing this mass, and what is called the "moment of interia" in physics will save energy for moving the car instead of the accessory. Thus the mileage increase.


The only accessory that is practical to get rid of on a street driven Jeep is the belt fan. I just leave my a/c off 75% of the time because I know what a drain it is. I actually turn my a/c ON when going down steep hills on the trail and highway because it helps with engine braking quite a bit.
The impact on fuel mileage due to the "moment of interia" of the fan is so low that it is meaningless to this discussion. If you have calculations or information to the contrary, it would be interesting to see. During highway cruise, fan RPM is relatively stable, so the "moment of inertia" does not apply. During normal street driving, it is likely that your Jeep is in an accelerating mode less that 10% of the time.

The mileage penalty due to the fan is mainly from the work performed; pushing or pulling air.This has an impact on fuel mileage, but so does the increased drag on the alternator attributed to an electric fan. When you replace the engine driven fan with an electric, both have inertia to overcome and both require energy to push or pull the air. The electric fan is typically lighter and slightly more efficient. The savings in fuel is not the total drag of the engine driven fan, but the difference between the two.

Drag racers realize a gain in power by switching to an electric fan, water pump, etc. but they are running off battery power which is recharge after the run. We have to constantly replace the power consumed by electric accessories. There is no free lunch.

[seanof30306@yahoo.]

Any belt driven accessory costs horsepower and mileage, every last one of them. Water pump, alternator, power steering, fan, and certainly the a/c (when on). They all have mass that must be accelerated along with the rest of the drivetrain when the rpms pick up; not to mention the energy they suck up to do the work they were designed to do. Reducing this mass, and what is called the "moment of interia" in physics will save energy for moving the car instead of the accessory. Thus the mileage increase.


The only accessory that is practical to get rid of on a street driven Jeep is the belt fan. I just leave my a/c off 75% of the time because I know what a drain it is. I actually turn my a/c ON when going down steep hills on the trail and highway because it helps with engine braking quite a bit.

Regardless, my point is simply thet there is no way you'll see a 1.5mpg gain from switching to an electric fan. You realizing you're talking between an 8 and 10% increase on most XJs? No way.

[kleake]

I have to agree that the gains people are seeing are that big... I get about 16-17 mpg with mostly highway driving... I know I could get better if I lighten my foot,,, but I have a hard time doing that... I do find it hard to believe someone getting over 20 out of an xj,,,, wonder if everyone is figuring it their gearing and such correctly.

Does anyone know what the XJ's were rated from the factory?

On a side note, my dad went from an XJ to a Toyota Prius a couple of years ago. He now has almost 50K on it in the 2.5 years and went from 18mpg up to 51-52.... The car cost him about 25K, which at the time a comparable Camery was going for just about 22-24K so the price increase wasn't that bad... And until you ride and drive one,,, they do look funny and seem like you are losing so much... This thing is every bit as fast as the XJ, has traction control, GPS navigation, fully electronic everything... Very impressive how much is in the car for the price, and to see he can get close to 600 miles out of his 11 gallon tank is pretty good....

XJ 300 miles at $3 gallon = $53.... Prius 300 miles at $3 gallon = $18..... hmmm,,, $35 savings... Considering I fill up almost twice a week, that would save me $280 a month in fuel, almost a car payment!.. And, there is one more thing you have to consider.... You may only save $1000 per year out of pocket, but if you are putting it in the pump it's gone forever, if you are putting towards a vehicle,, you will at least be investing it and getting some of it back when you sell the car.... I understand the value of the car will go down considerably,,, but at least you do get something back,, unlike trying to sell your used fuel.....

[j99xj]

Fans aren't needed (either type) when the vehicle is moving forward at a moderate to fast speed. There is more than enough air rushing past the radiator/engine from just the motion of the vehicle itself.

The stock belt fan runs ALL the time, when it is needed or not.

The electric fan is only run when needed if you hook it up in tandem with the stock electric fan. (slow speeds around town, rock crawling, etc.) It shuts off completely once the vehicle speed increases and the "ram air" effect takes over. This is where most of the mileage gain is found. The other part of the mileage gain (as stated before) is from the omission of the shear mass of the stock belt fan. (The moment of inertia arguement that I said earlier is valid.)

Running a belt fan when it is not needed, is like sleeping with the lights on. It just doesn't make sense.

[MaXJohnson]

Fans aren't needed (either type) when the vehicle is moving forward at a moderate to fast speed. There is more than enough air rushing past the radiator/engine from just the motion of the vehicle itself.

The stock belt fan runs ALL the time, when it is needed or not.

The electric fan is only run when needed if you hook it up in tandem with the stock electric fan. (slow speeds around town, rock crawling, etc.) It shuts off completely once the vehicle speed increases and the "ram air" effect takes over. This is where most of the mileage gain is found. The other part of the mileage gain (as stated before) is from the omission of the shear mass of the stock belt fan. (The moment of inertia arguement that I said earlier is valid.)

Running a belt fan when it is not needed, is like sleeping with the lights on. It just doesn't make sense.
ya got that "shear mass" calculated yet???

do ya think that "ram air" effect might make the fans job a little easier???

[j99xj]

ya got that "shear mass" calculated yet???

do ya think that "ram air" effect might make the fans job a little easier???

1. Dr. Dyno says the belt fan is 5 lbs. I believe him.

2. Why yes, thats the entire point of an electric fan. If we utilize the "ram air" effect at moderate to high speeds, the fans job is easier because its simply not running, thus the engines job is easier (doesn't have to drive it), thus you save gasoline!

If you believe the "ram air" effect makes the fans job easier, you MUST believe the electric fan saves gasoline! And that figure is about 1.5 mpg.

I don't know if you being a smart ass or just plain stupid, I can't explain this any more clearly.

[XJBANKER]

i've got 33's with 4:56 Gears and can leave layton utah and arrive in Moab on 1 tank of gas. As long as I can get to moab I am happy. I am too excited to stop anywhere on the way. Stopping the Jeep on its way to Moab is like stopping a dog without a leash from chasing a stray cat. It knows what it wants and it is going to get it.:soapbox:

[seanof30306@yahoo.]

Fans aren't needed (either type) when the vehicle is moving forward at a moderate to fast speed. There is more than enough air rushing past the radiator/engine from just the motion of the vehicle itself.

The stock belt fan runs ALL the time, when it is needed or not.

The electric fan is only run when needed if you hook it up in tandem with the stock electric fan. (slow speeds around town, rock crawling, etc.) It shuts off completely once the vehicle speed increases and the "ram air" effect takes over. This is where most of the mileage gain is found. The other part of the mileage gain (as stated before) is from the omission of the shear mass of the stock belt fan. (The moment of inertia arguement that I said earlier is valid.)

Running a belt fan when it is not needed, is like sleeping with the lights on. It just doesn't make sense.

Nobody's arguing those points, dude. The point that sticks is that the fan is costing 1.5 mpg. Now, you're making an emphatic statement, and you're arguing it's true when others are skeptical; yet you have no data to back it up.

I can comfortably say the throttle body spacer and removing the silencer are NOT worth the 1.55 mpg you said they are. I've done them on my car and saw no change.

As far as the fan goes, I haven't done it on my Jeep, but I have done it on another car, with a MUCH bigger fan (and therefore much more drag from that fan), and saw nowhere near the 1.5 mpg increase you're insisting on. Now, if you're going to argue the point, then come back with some DATA: go drive a 60 mile course with the fan on, then pull the fan and drive it again, recording the mileage both ways. Even as hot as it is, if you do it at night, it won't be an issue. If you're not willing to back your s*it up with actual testing, then you need to just be quiet.

On another subject, how about using a vacuum gauge to help alter driving style? The more throttle you use, the lower the vacuum drops, right? You could put a vacuum gauge in a pod and watch it to see where your foot's the heaviest, then working on taking the lead out.

[MaXJohnson]

...I can't explain this any more clearly.
Apparently not.

At this point, I can return your personal insults, or assume that you don't have the depth of knowledge to back up what you've read on the internet

[BrettM]

take it easy on the gas pedal. it's proven, and free.

[MaXJohnson]

take it easy on the gas pedal. it's proven, and free.
as well as the pedal on the left; designed to turn kinetic energy into heat ;)

[j99xj]

On another subject, how about using a vacuum gauge to help alter driving style? The more throttle you use, the lower the vacuum drops, right? You could put a vacuum gauge in a pod and watch it to see where your foot's the heaviest, then working on taking the lead out.

It's a good idea that Jeep thought of. My 99 xj has a overhead console computer that has a real time mpg mode and a running average. This will come in handy for my future mpg experiments that were requested by popular demand.

Anyway, driving habits are the first thing I talked about on this post. They truly have the most effect on mpg. I drive the speed limit or slightly less than the speed limit, and I accelerate as smoothly as possible. I also let up on the gas pedal when my rpms reach 2000 so the tranny will shift to a higher gear. With stock gears and slightly larger than stock tires, I can accelerate to 65 mph without the rpms going over 2000 rpms at any point. It takes longer but the rewards are there.

Fans are not created equal. Diameter is only one parameter. Number of blades and the blade pitch are all important factors in how much air a fan moves. So the belt fan you replaced on your car was bigger diameter than the Jeeps belt fan, but the blade pitch and number of blades are almost certainly to be different. Those can lead to false impressions. Bigger doesn't always mean more. The diameter of the pulley also has a huge effect on fan speed. The Jeep pulley diameter and the cars diameter are almost certainly different.

Look at props on boats. Even if two props are the same diameter they could be designed for different purposes, (waterskiing, vs. trolling). And those design differences come from the blade pitch.

[j99xj]

[quote=j99xj]This will come in handy for my future mpg experiments that were requested by popular demand. [\quote]

I'm changing my mind about running the before and after tests. I really don't think its needed.

Dr. Dyno got 1.4 mpg http://www.angelfire.com/my/fan/electric.html
GoJeep got a 5% increase with amounts to about 1 mpg. http://www.go.jeep-xj.info/HowtoElectricFan.htm

These folks are the most credible Jeep maniacs on the internet. The average of the numbers is 1.2 mpg.

This is lower than my 1.5 mpg figure. But it is not a bad estimate.

This guy got about 2 mpg on his f150. http://www.obd-codes.com/howto/electricfan.php

[Big Red]

I know results are greatly influenced by your driving style, but I have a 98' 2 dr, 5 speed, 231 XJ and a 96' ZJ fully loaded limited with a V8 and NP249 AWD. I know that the ZJ is a gas hog averaging about 13-14 MPG with street and hwy driving to work 15 miles each way. The 98' XJ is sitting and I have it listed for sale, but I'm thinking strongly of using it as my commuter dd. I already have my 91' 2 dr wheeler, and like 2 dr XJs.

What kind of gas mileage are you guys getting on a stock 5 speed XJ typically? Does 18-22 MPG seem reasonable? Do manuals do better with their 3.07 gears compared to the autos with their 3.54 gears? I drove the XJ cross country from Northern IL to Sacramento, CA and I didn't keep track of the mileage, but I thought I did pretty good about 20 MPG or so, maybe more. I know the XJ is quite a bit lighter than my 96' ZJ Limited, how much I don't know, but I think it's 700 lbs or so, maybe more.

If I keep the ZJ, I'm probably going to take off the front driveshaft, now on 93-95' ZJ with NP249 this will damage the VC, but on 96'+ the NP249 saw a improvement and it shouldn't hurt it from what I read on the ZJ site NAGCA.com, what do you guys think? Can I see about 2 MPG increase if I take off the front driveshaft so I don't have AWD? I don't need it here in Sacramento, especially not in the dry summer.

Troy

[streetpirate]

I'm able to get 19.8mpg with city/highway mix 3.55's and 26.4in tire.
and it gets it to the floor every day.

right now i cruise at 2250 at 70,

would the mileage have a huge change if i regeared to run 2700ish at 70?

[Big Red]

I'm able to get 19.8mpg with city/highway mix 3.55's and 26.4in tire.
and it gets it to the floor every day.

right now i cruise at 2250 at 70,

would the mileage have a huge change if i regeared to run 2700ish at 70?

I don't know how huge, but 2700 RPMs is getting up there. I try to shoot to staying under 2500 in my final gear. I can do this with my 91' XJ 4.0L/AW4 on HP44/9" and 37's with 5.43 gears in the 9" doing 70.

I'm still learning my 5 speed 98' XJ 4.0L 3.07 geared dd XJ. I'm doing 90 at about 2600-2750 RPMs or something like that. I'm going from 215 tires to 30" BFG Long Terrains, but I still hope to get at least 15 MPG city/hwy & 19 or so strictly hwy, I'd be happy with this over 13 MPG if I was lucky city/hwy with my V8 AWD 96' ZJ I just sold.

Troy

[j99xj]

I think I figured out why the mpg of people on this forum are so polarized. I think its because the people that have upgraded tires sizes have not changed the speedo gear in the transfer case.

In other words, when the odometer reads 1 mile more than before, the people with bigger tires have actualy traveled more than that distance. This will seem to lower the mileage for those people.

About a year ago when I bought my new tires I went about an inch higher than stock. Now that is not a huge difference, but since I travel at relatively high speeds on the interstate (75 mph) I wanted the speedo to be as accurate as possible, so I got a different speedo gear. I found a chart that has a list of tire sizes and the correct speedo gear for that size. I don't know exactly where that chart is but I'm sure its around somewhere on this www.

[jeepdude10000]

distance per time driving also changes MPG. I got a new job, within 3 miles from home, My last job was 12 miles from home, I use to get 23 MPG to and from work and now I get only 16 MPG.

Oh yea I get to play with a big 4X4 now on the new job she only hits 26 mph but she has grunt.

http://baggernaddel.feischo.de/kipper/images/CAT-D25D-006.jpg

[Dr. Dyno]

I'm changing my mind about running the before and after tests. I really don't think its needed.

Dr. Dyno got 1.4 mpg http://www.angelfire.com/my/fan/electric.html


The mpg gains occur mainly during stop/go city driving where an electric fan is needed most and a clutch fan would be working hardest. On the highway where you have enough ram air cooling of the radiator, you don't need a fan and the clutch fan is virtually inactive anyway so mpg gains would be minimal.