Daily Driver: Improving MPG without going crazy

Here's my XJ

96 Cherokee Classic 4.0
95 head: intake and exhaust ports matched to gasket; otherwise stock
Up Country 3.55 posi
255/70R15 Bridgestones
81,000 miles
15 mpg in town, 17 highway

I've recently gotten the MPG up to 20 by using some hypermiler driving techniques. I use my XJ exclusively on the road. I want to keep it because a) it's paid for, b) it's really good in the snow and rain, and c) I like it.

I'd like to look into making some improvements to increase mileage, but, I don't want to be stupid in doing so. An increase of 3 mpg, for example, saves me about 220.00 per year with 3.00 per gallon gas. If I spend 400.00 to get that increase, it'll take me two years to save any money.

One area I've been looking at is the gearing. My rearend has been howling since the day I bought it new, and it started making a moaning sound on very slow speed corners about three years ago. It used to do it only after long highway runs, but, now, it's doing it all the time. I figure it's the clutches in the posi.

When I go into the rearend, IF the gears have to be replaced, I'm considering going to a 3.08. I figure it'll cost another 2-300.00 to do the front as well. If I don't have to replace the gears in the rearend, I don't think swapping ratios would make sense. Unless going from 3.55s to 3.08s would make a HUGE difference in mileage, it just wouldn't make sense to spend 600 bucks to do it.

I'm wondering what kind of difference I could expect in mileage from that gear swap, and am also looking for suggestions on other mods that would improve mpg.

swap axles with a jeep with a 5spd. Probably get them for next to nothing and you wont have to buy gears and pay to have them set up.

You'd be going to 3.07s and I don't think you'd notice a difference in mileage.

When was the last time you replaced the O2 sensors, cleaned the injectors and the throttle body? What kind of air filter do you use? How about fluids? When was the last time you changed your diff fluid? I'm guessing you have an auto tranny, since you have 3.55 gearing...?

KSXJ said:

swap axles with a jeep with a 5spd. Probably get them for next to nothing and you wont have to buy gears and pay to have them set up.

Click to expand...

Yeah, I've thought of that, but, I've had a search set up on ebay for over 6 months, and haven't seen any. What's a good source?

local jeep clubs, here, local 4x4 shops, junk yards

JNickel101 said:

You'd be going to 3.07s and I don't think you'd notice a difference in mileage.

When was the last time you replaced the O2 sensors, cleaned the injectors and the throttle body? What kind of air filter do you use? How about fluids? When was the last time you changed your diff fluid? I'm guessing you have an auto tranny, since you have 3.55 gearing...?

Click to expand...

Yes, it's an auto.

Cracked the head at 70,000 miles when it overheated. Had injectors refurbished and balanced, new Bosch o2 sensors, new exhaust manifold to replace cracked one, new filter & fluid in tranny.

Stock air filter, but, I drilled some 3/4" holes in the fender-side and bottom of the air box

Never changed diff fluid, but, have checked it myself and had it checked by a local differential guru. No burnt smell and good viscosity. There has been a problem with the rearend since I bought it. The only XJ I'd been in before I bought mine was a 2wd. When I got mine, I noticed right away that it howled, but, it sounded a lot like my old CJ with the 32s did, and I assumed it was supposed to whine. It was several years before I rode in another 4wd XJ and found out mine wasn't supposed to be that noisy. The warranty was out by then, and it has never gotten any worse. I assume the whine is an axle bearing, but, I'v always been puzzled that it has never gotten any worse.

The clutches slipping (I assume) started at around 50,000 miles and has gotten progressively worse.

As far as swapping gears, I wonder if the carrier(s) in there now would even work with 3.07s? On GM 9 bolts, 10 bolts and 12 bolts, the carriers aren't the same.

waaaait.

Going to 3.07's will *NOT* increase your gas mileage, but hurt it.

The 4.0 does not like to run at low RPM's, you have to throttle down harder to get the same momentum, and burn more gas in the process. A jeep on 32" tires will get 12-18 % better gas mileage with 4.56 gears (lower final ratio than stock) than it will with 4.10 gears (higher final ratio than stock).

If you want to adjust for 29.2" tires with gears, you should be looking at going the other way, to 3.73's.

3.07s meant that I had to hang my foot into the gas pedal at all times.

4.56s keep the RPMs up and in the power band, letting me use less skinny pedal for even better results.

You're absolutely going to hurt your MPG by going to 3.07 gearing.

I think if you're that worried about gas mileage and 20 MPG isn't good enough, then maybe it's time to find another means of transportation. Some of those import 4wd vehicles probably get good gas mileage if you're looking for a reliable DD that does well in rain and snow.

cal said:

waaaait.

Going to 3.07's will *NOT* increase your gas mileage, but hurt it.

The 4.0 does not like to run at low RPM's, you have to throttle down harder to get the same momentum, and burn more gas in the process. A jeep on 32" tires will get 12-18 % better gas mileage with 4.56 gears (lower final ratio than stock) than it will with 4.10 gears (higher final ratio than stock).

If you want to adjust for 29.2" tires with gears, you should be looking at going the other way, to 3.73's.

Click to expand...

I'm assuming 29.2" is the height of the 255/70 tires?

Shouldn't I have then seen a drop in mpg when I went from the stock 225/75s to the 255/70s? I didn't, although, I also didn't have the speedometer calibrated. The guy at the tire store told me the OBDII pcm would adjust for the difference, but, I've never heard of one being able to do that automatically.

An electric fan set-up, like the popular Ford Taurus unit, is a good way to get a little more snap and massage a little more fuel economy from your Jeep.

Remove the roof rack; if you never use it, you won't miss it, and it'll cut down on drag.

A cold-air intake can be fabbed up for relatively cheap. $70 for the cone filter, some stainless exhaust pipe, and some sheet metal for the heat shield.

Remove any skidplates, hooks, or other heavy XJ parts not needed if you're not travelling offroad.

Run a smaller tire size, say 225/75/15; it'll cut down on rolling resistance, and the lighter wheel is easier for the motor to spin.

Check out this site, http://jeep4.0performance.4mg.com/ it's got neat tech info, and ways to save gas.

seanof30306@yahoo. said:

I'm assuming 29.2" is the height of the 255/70 tires?

Shouldn't I have then seen a drop in mpg when I went from the stock 225/75s to the 255/70s? I didn't, although, I also didn't have the speedometer calibrated. The guy at the tire store told me the OBDII pcm would adjust for the difference, but, I've never heard of one being able to do that automatically.

Click to expand...

No, the PCM does not auto adjust for your tire size, you have to change the speedo gear.

You didn't make a big differnece in tire size before, going from just under 28 to just over 29, but doing that *plus* going to higher gears and you will start to notice a loss in mileage.

darwin said:

3.07s meant that I had to hang my foot into the gas pedal at all times.

4.56s keep the RPMs up and in the power band, letting me use less skinny pedal for even better results.

You're absolutely going to hurt your MPG by going to 3.07 gearing.

I think if you're that worried about gas mileage and 20 MPG isn't good enough, then maybe it's time to find another means of transportation. Some of those import 4wd vehicles probably get good gas mileage if you're looking for a reliable DD that does well in rain and snow.

Click to expand...

The Jeep is paid for, and I like it a lot. Any increase in mileage I got from buying another vehicle would be more than offset by the aquisition cost, and there isn't another vehicle I'd enjoy as much as my Cherokee. I'm 10 years+ into it, and still get a kick out of it.

There's nothing wrong with trying to see if I can get better mileage. It doesn't mean I don't like it, or I'm "disloyal". I've looked online to see how much a boob job would be for my wife, too. Doesn't mean I'm ready to dump her and run off with a stripper.

XJPhoenix said:

An electric fan set-up, like the popular Ford Taurus unit, is a good way to get a little more snap and massage a little more fuel economy from your Jeep.

Remove the roof rack; if you never use it, you won't miss it, and it'll cut down on drag.

A cold-air intake can be fabbed up for relatively cheap. $70 for the cone filter, some stainless exhaust pipe, and some sheet metal for the heat shield.

Remove any skidplates, hooks, or other heavy XJ parts not needed if you're not travelling offroad.

Run a smaller tire size, say 225/75/15; it'll cut down on rolling resistance, and the lighter wheel is easier for the motor to spin.

Check out this site, http://jeep4.0performance.4mg.com/ it's got neat tech info, and ways to save gas.

Click to expand...

I would agree with removing an aftermarket roof rack to cut down on mileage, but the rest of that I think makes no difference. The modified intake won't be smog legal in most states either.

The stock XJ intake *is* a cold air intake, so your not going to improve it. It already breaths better than the 4.0 can use, why change it?

I get 20 mpg on 35" tires with aftermarket skids, front and rear bumpers and a winch. I got 20 mpg before I installed those.

If I put my surco roofrack on, I can see a drop in mpg right away.

Run a can of BG44K through the gas every 10k miles will keep the injectors clean and cut the carbon.

The single best mod is a tire gauge. Keeping the tires properly inflated will help more than anything.

Synthetic in the Diff's and Tcase will help a bit. Keeping a clean air filter is also a must.

seanof30306@yahoo. said:

The Jeep is paid for, and I like it a lot. Any increase in mileage I got from buying another vehicle would be more than offset by the acquisition cost, and there isn't another vehicle I'd enjoy as much as my Cherokee. I'm 10 years+ into it, and still get a kick out of it.

There's nothing wrong with trying to see if I can get better mileage. It doesn't mean I don't like it, or I'm "disloyal". I've looked online to see how much a boob job would be for my wife, too. Doesn't mean I'm ready to dump her and run off with a stripper.

Click to expand...

This thread gets made five times a week on every single Jeep board. Whenever someone tells the OP the logical thing (If you want 30 MPG, buy another car) they get all defensive like you did. It's not an attack, it's just facts. No one called you disloyal... It's a car.

You're not going to get 30 MPG out of your XJ. You're lucky to hit 20 MPG.

Enjoy it, drive it gently and get your 20 MPG.

old_man said:

Run a can of BG44K through the gas every 10k miles will keep the injectors clean and cut the carbon.

The single best mod is a tire gauge. Keeping the tires properly inflated will help more than anything.

Synthetic in the Diff's and Tcase will help a bit. Keeping a clean air filter is also a must.

Click to expand...

This is your best bet. The tire PSI is the most important.

Come on guys, I can't believe no one suggested getting a tornado and a fuel line magnitizer!


Or how about ripping out all the extra seats and carpet to cut down on weight? And lower it and put on air dams to lower drag? Convert to a single wiper blade (yellow) while your at it. And a high-flow muffler with a 6" tip will help too.

darwin said:

This thread gets made five times a week on every single Jeep board. Whenever someone tells the OP the logical thing (If you want 30 MPG, buy another car) they get all defensive like you did. It's not an attack, it's just facts. No one called you disloyal... It's a car.

You're not going to get 30 MPG out of your XJ. You're lucky to hit 20 MPG.

Enjoy it, drive it gently and get your 20 MPG.

Click to expand...

I don't recall saying I was looking for 30 mpg.

And, if these questions come up so often, that would indicate there are a lot of people interested in that information. On every other forum I'm involved in, that interest would lead to a sticky.

And, I apologize. I'm apparently under the mistaken impression that special interest automotive forums are places where people discuss ways to improve the looks, performance, efficiency, etc. of their vehicles; where people who know more share their knowledge with people who know less. I didn't realize that you had to just accept what you had, that you dare not ask a question, and, if you want improvement, you have to sell your vehicle and go buy another.

Again, I'm sorry. I have to go now so I can log onto the F-Body forums I belong to and tell everyone they're wasting their time and money, that they have to buy new cars that already do what they've been trying to get their Camaros and Firebirds to do, and log onto the poker forums I participate in and tell everyone to immediately stop trying to learn more about the game, that they simply have to accept the level that they play the game at now, or go play another game, etc.

Ignore the people who slam every question people post or make a post bitching about using the "search" function. Some of us actually try to help, and take the same amount of time it would take to make a helpful answer, instead of making a smartass one.

Most have made good suggestions. I'd still change your diff fluid, didn't you say you were noticing excess noise from your rear? It should be changed every 10-12,000 miles. Check for metal shavings in your rear.

Electric fan idea was good too...forgot about that...

People will argue left and right about what different mods do....for me, I noticed a difference in power and mileage when i switched to a hi-flow cat and magnaflow muffler....but I also have a K&N FIPK, so....
If you decide to go with a new intake, don't get a metal one. Metal conducts heat a LOT better than thick, black plastic, so switching to something with a metal intake tube wouldn't help, would probably hurt.

I have a 01 auto trany and 3.07s gears with 235 tires.
The 1/4 mile time is up the 1/8 mile is unchanged but it fells slower.
You may never feel the difference if you don't drive the engine harder to make up for the loss of gearing. Not ever start needs to be wide open and this is when you will really feel the difference. (IMO)
Also with a light foot you will not see a drop in city MPG nor will you see an improvement in town.
On the highway at 55 or better you may see 1 MPG improvement.
How you drive and were can make all the difference in if a setup works or not.
I also have a 01 5 speed with 3.7 gears and 265 tires. At 45 MPH (most of my driving) I am using 5th a lot more then before regearing. So my over all final drive ratio is about the same at 45 MPH so are my RPM so is my MPG. This time the set up worked for me.

01 4 door all stock. I average 17 mpg between intown and freeway. Thats with or without AC. I dont do fast take off's nor do I drive like a little old lady. I am just settled into the fact that 17 is what I get and deal with it. I would rather get 17 and know I will more than likely get 300,000 miles out of her, than 25mpg and 150,000 miles. I also have a 90 with 6" lift, 410 gears and 33" tires with over 301,000 miles on her and she gets 15 mpg.

There is nothing wrong with wanting more mpg, but there is a line that cannot be moved, and I think that the 17 - 20 mpg is the limit no matter what you have on it or how you drive.

Also, just ignore the guys that try to make you feel like crap for bringing up a disussion. I dont see anything wrong with asking about it and seeing what others are getting with there jeeps. You just never know what you might find from some redneck hillbilly or bean counting city slicker that got them a little more juice.

Sincerely, a bean counting redneck..........LOL

Lata