gas mileage

[xL8 APEKSx]

Man, there really seems to be a descrepancy with fuel economy!

My 2001, completely stock, with 75k miles is lucky to run 15mpg highway. As a matter of fact, my 1993 Grand Wagoneer with full-time 4WD and a 5.2L got better mileage than my little 3000lb XJ.

I see countless posts about poor fuel economy, with no apparent fixes. Seems to me that if the window sticker says 15 city/20 highway, you should be able to at least come within the ballpark.

Otherwise, Jeep should be paying to fix the problem IMHO.

On a related note...how much fuel is supposed to be left in the tank when the idiot light comes on? Is is something like 2 gals? I know the tank is 20 gals...

 

[J-Roc]

window stickers are always bullshit, I had a jeep dealer try to convince me that his daughter was getting 32mpg on the highway in a brand new '07 Rubi...

I run 31's a light foot and I see about 10mpg in town, a bit of it has to do with winter gas, and a large portion of it is the fact that something in my system is buggered up.

I'm running cheap ass copper plugs with brand new ignition components, amsoil air filter, but my 02 sensors and cat are original, which I am suspecting are the problem however I only have 120k on the clock

As for fuel, when the light comes on I can fill it with about 60L, about 16 gallons

 

[Del00XJ]

Hell, Rusty's 8'', 35'' Mall Terrains, and 4.56's... , I just drive. Haven't really checked. I know it isn't good. Over the summer, I rode to the beach with the kayaks on the roof. 3/4 of a tank for a 120 mile one way doing around 70ish...

 

[jeepboy381]

I love reading these gas mileage post. People post up saying how they lifted their XJ and put 31's on it and are getting upwards of 16mpg city. I highly doubt it!!!! I have three Xj's which I am anal about maint. on and I have liffted all three of them. Here is what I have learned from personal experience.

97XJ auto 4.0 3.55 gears
Was getting around 18-21 mpg stock, put on 30in BFG A/T's and am now getting 14-16 mpg average with mixed city/highway.

2000XJ auto 4.0 3.55 gears
I was also getting 18-21 and on highway trips I could pull off 22-23mpg. I put 31's on it and am now doing mostly backroads driving at 45-55mph. Now I get anywhere from 13-15mpg and I am gussing highway flat ground I might average 16mpg but have yet to do it. This XJ has all new 02 sensors spark plugs and frequent oil changes. I am in the process of putting exhaust and intake on it to help with econony. 4.10 gears in the near future as well.

1987 XJ 4.0 5 speed
I believe stock I was getting 21-23 mpg mostly highway and some bad traffic. Now it is on 33 in BFG M/T's with 4.56 gears and I average 16-18 mpg if I treat it good and don't wheel it. Now the only way I get decent mileage in this thing is for one it is a 5 speed, and for two it has gears for teh larger tires.

I would not get your hopes up on getting 16mpg or higher with 31's and stock gears. Maybe if you drive in a perfect world where you can set cruise control and only drive on flat ground will you get much better. I would expect 13-15 with an automatic XJ driving it normal. Just from personal experience and everyone else I have talked to. If you want halfway decent mileage budget in for gears as well, this is teh singel most important mod to get your mileage back.

 

[Tenny]

'94 Auto 4.0 3.55 31x10.50x15's No load (other than tool box / etc in back) I'm getting 13.8mpg on average (corrected)... New O2, New Intake, recent plugs / wires / dist.

I learn to just accept it...

 

[CJ's98XJ]

What is the RPM at 65 mph for 31's and 4.10 gears???

I run stock gears with 31's now. My commute is almost all cruise control driving at sea level and the closest thing I see to a hill is a bridge over the ICW. Seems to me that the 31's actualy improved my mpg, kinda like a big overdrive gear.

Does this make sence to anyone else? or am I sucking up extra gas relying too much on torque to push the Jeep along at 65 mph (2000-2100 rpm)?

 

[RªMB°]

I Put New 31x10.50 BFG M/Ts On My Jeep And I Can Do A 41Mi Round Trip Off An 1/8 Tank If I Baby It And Drift, Slow Acceleration, ETC. The Trip On Stock Tires Ate 1/4 Tank Irreguardless Of Driving Conditions Go Figure.

 

[clunk]

just did a 500 mile highway trip in my 1990 with around 50 miles of that being stop and go driving and calculated my mileage, came in at just over 25MPG running with the AW4 in "power" mode exclusively. Apperantly switching to comfort mode is supposed to net another 1-2 MPG's, but I don't like the lack of power going over mountain passes in comfort mode.

I have LT235 all terrains, stock height, new exhaust (OEM replacement, not performance), recently replaced the air filter as well..the plugs, wires, cap and rotor and that were (possibly) replaced by the previous owner..but I'm really not sure since I don't have any maintenance records.

Big key is keeping good pressure in the tires, I run them at 32psi normally but bumped them up to 33psi before the highway trip (the original glove box manual I got with my jeep suggests going up to 33-34psi for extended highway driving). My mileage was falling off a bit until I checked my tire pressure..since it started getting cold the pressure dropped quite a bit..I was unkowingly running at 27psi all around...not good for fuel mileage! Brought the tires back up to spec and saw the increase in MPG's right away.


All that being said I'm still lucky to get 17mpg's around town, most of my city driving is really short trips though, so that degrades fuel mileage instantly.


Oh, and before you guys all have a panic about my 25mpg's...that's 25mpg's here in Canada, so I'm factoring in the imperial/metric conversion. I don't know what that works out to exactly but I'd guess it's closer to 21mpg's for you guys down there.

 

[xL8 APEKSx]

If you do a lot of highway driving, wouldn't larger tires and STOCK gears INCREASE your fuel economy, as long as you stayed within a certain speed range?

Larger tires = lower RPM's for the same MPH compared to smaller tires.

So as long as you don't pass the threshold where aerodynamic inefficiency takes over, I would think 55-60mph with larger than stock tires would net an increase in fuel economy (on the highway...city driving would suffer as more force is reqiured to get the vehicle rolling).

Shorter gears (ie, from 3.55 to 4.56) would spin faster and raise the RPM's again in favor of shifting the TQ band down, which would hurt high-speed freeway mileage in favor of more TQ when wheeling (and less stress on the driveline when wheeling).

...there are other factors like lift and coefficient of drag, so the only way the above would NOT be true is if the XJ is so aerodynamically inefficient that RPMs and MPH are almost irrelevant altogether.

A smooth belly pan under most of the vehicle would make a noticeable increase in fuel mileage, but I've only been underneath mine once for about 10 secs so I haven't even noticed if it would be possible. But a little sheet metal could save a few precious bucks per tank I bet. Worked on my Volvo...another 1-2mpg (and 10-20 miles per tank) just by adding a pan to the rear 1/3 of the car.

 

[Starscream]

If you do a lot of highway driving, wouldn't larger tires and STOCK gears INCREASE your fuel economy, as long as you stayed within a certain speed range?

Larger tires = lower RPM's for the same MPH compared to smaller tires.

So as long as you don't pass the threshold where aerodynamic inefficiency takes over, I would think 55-60mph with larger than stock tires would net an increase in fuel economy (on the highway...city driving would suffer as more force is reqiured to get the vehicle rolling).

Shorter gears (ie, from 3.55 to 4.56) would spin faster and raise the RPM's again in favor of shifting the TQ band down, which would hurt high-speed freeway mileage in favor of more TQ when wheeling (and less stress on the driveline when wheeling).

...there are other factors like lift and coefficient of drag, so the only way the above would NOT be true is if the XJ is so aerodynamically inefficient that RPMs and MPH are almost irrelevant altogether.

A smooth belly pan under most of the vehicle would make a noticeable increase in fuel mileage, but I've only been underneath mine once for about 10 secs so I haven't even noticed if it would be possible. But a little sheet metal could save a few precious bucks per tank I bet. Worked on my Volvo...another 1-2mpg (and 10-20 miles per tank) just by adding a pan to the rear 1/3 of the car.

If you were in, like, TX I could see that. With no hills and the wind to your back I mean...

 

[xL8 APEKSx]

...or SoCal. There's no hills near me at all, just freeway. I'm 1 mile from Disneyland. hehe.

 

[Starscream]

...or SoCal. There's no hills near me at all, just freeway. I'm 1 mile from Disneyland. hehe.

Either way, I'm on 32s and on the highway in 3rd gear (not overdrive) I spin 2750 RPMs @ ~70mph. I still get like 16/17mpg, probably 12/13 city. Shit sucks.

 

[mitsumotors]

from what i understand, gas mileage is not directly linked to your rpm like that. you could be getting better gas mileage at higher rpm in some gears and worse mileage at really low rpm. if i drive 65mph in fifth gear i rev at like 1600rpm, but if i just stay in fourth i rev at like 2200rpm and get better gas mileage.

 

[nturboy12]

I would like to know the secret I get 11to12 mpg 89 new motor less than 5k, 4.88, 33s at 500 miles old at 32 psi ,6inch lift new wires ,plugs cat oxygen sensor so full tune up and an,easy foot with a lot of coasting to signals all city driving

 

[mk153smaw]

One thing that helps me, I removed the air link from the valve cover to my intake. Now the fouled air does not enter my intake sysyetm. I am working on an oil/air serperator setup, so water doesnt foul up everytime I submarine it, and reconnect it back into the intake.

 

[IslanderOffRoad]

What is the RPM at 65 mph for 31's and 4.10 gears???

I run stock gears with 31's now. My commute is almost all cruise control driving at sea level and the closest thing I see to a hill is a bridge over the ICW. Seems to me that the 31's actualy improved my mpg, kinda like a big overdrive gear.

Does this make sence to anyone else? or am I sucking up extra gas relying too much on torque to push the Jeep along at 65 mph (2000-2100 rpm)?

I seem to be around 2500rpms at 70 with 4.10's and 31's on a 5spd.

 

[xL8 APEKSx]

from what i understand, gas mileage is not directly linked to your rpm like that. you could be getting better gas mileage at higher rpm in some gears and worse mileage at really low rpm. if i drive 65mph in fifth gear i rev at like 1600rpm, but if i just stay in fourth i rev at like 2200rpm and get better gas mileage.

Well, there are certainly more variables...but as a general rule, the higher you rev an engine, the more fuel it will consume.

Another big thing to consider is 32" A/T's at 40psi are not at all the same as 32" Swampers at 35psi. These would net very different fuel numbers even though they are technically both 32's.

After resetting my P0455 CEL and fixing the problem, airing my tires to ~40psi, and realizing I still have 3.5 gals in the tank when my gas light comes on...it turns out I'm averaging 18mpg mixed city/hwy in my stock 2001. :cheers:

 

[Terrific]

Ouch!!!! Ive got a 98 w/RE 3 1/2", 31x10.50's on 15" steelies and dick Cepek FC2's and stock gears. I'm getting about 13 mpg. I'm having a new house built in NC and my commute is gonna be bout 45 minutes. this mileage is gonna kill me. What steps would be good to get some better milage????

I have Black steelies, 31 Pro Comp MT's, and 4.10 gears. I get a bout 16.3mpg overall for the last 6 months. The Rock crawler steel 15x8's are much heavier than the stock rims. Maybe stock rims would help fuel economy some over the much thicker and heavier aftermarket steelies. What do you think.

By the way, my 92 2wheel drive on 225's gets 20+ overall

 

[yossarian19]

I remember reading somewhere that peak fuel economy on a 4.0 is around 25-2700 and, though I rarely drive slower than 80 on the interstate (2700 RPM in AW4 4th w/ 30s), I think it has been working out for me. 19 MPG at altitude with a lot of mountain roads thrown in.
Check out this link:
http://www.4wheeloffroad.com/brandpages/jeep/131_0403_jeep_xj_cherokee_mileage/index.html
Their scientific method is questionable but the theory all matches up - they increased flow into and out of the engine and increased spark efficiency and recorded increased MPG as well.
I'm having trouble finding the exact post but according to 5-90 (yes, I'm citing him as a source) peak volumetric efficiency is at peak torque is at 2800 RPM. This means the most power per unit of gasoline is had at 2800 RPM which is not the same as saying it is the most MILES per gallon, but for highway driving it is very close.
People have had good results using the 99-01 intake manifold, as well.
Hope this is usefull info.

 

[yossarian19]

I would like to know the secret I get 11to12 mpg 89 new motor less than 5k, 4.88, 33s at 500 miles old at 32 psi ,6inch lift new wires ,plugs cat oxygen sensor so full tune up and an,easy foot with a lot of coasting to signals all city driving

Check the TPS - I've been hearing here adn there that the TPS turned out to be the magic bullet. It sounds like you've done everything else.