Fuel economy

I'm running 33's on a stock D35 and D30, my best guess is 3.XX something gears, I don't know for sure.

I've owned the XJ for about eight years now, originally it got about 17-20 MPG. The engine failed and was replaced with a remanufactured engine and got about 15 MPG on the highway. After installing the lift, I'm getting no better than 10 MPG highway.

I can kind of understand the differences between the stock engine and reman engine, with the pistons having to be bored out and what not, but I don't understand how that makes a 5 MPG difference...

I can kind of understand the added weight of the lift and tires sucking fuel, but not another 5 MPG difference...

I've read about builds with lifts larger than mine, where the user gets in the 17 MPG range. I also have friends who have tracked something along 6 MPG fully kitted out. Any of you more experienced gear heads have any tips or thoughts to clear up my confusion and improve the fuel economy of my rig?

Does the speedo read the correct speed - and therefor the correct mileage? If you upped the tire size and didn't change the speedo gear that will throw your mileage calculation off by ~17%.

The remanufactured engine should have no effect on your mpg.. Id assume if anything it would get a tad better. I would look elsewhere for the problem, unless the engine itself runs like crap.


FWIW:
I get 17.5 MPG highway with 4.88's, 33" tires, 4.5" of lift and a 5 speed. I got 15-16 highway with 31's and 3.07's, and 19 city with stock tires and no lift and 3.07's.

Plugged cat or lazy O2 could be a problem to. I ran stock gears on 33" for 8 mo. and hated it. One thing i came to terms with though before i re geared... If i wanted good MPG... I would have built something other than my XJ.

As stated above though.. Your speedo gear is wayyyyyy off.. Get rid of that D35 and re gear your axles. Get you odom.. back to stock settings with those 33"s. then start finding out what you MPG is.

As people have said. REGEAR. driving around on 33's and stock gears, your engine is always working hard - like you are towing a boat behind you ALL THE TIME.

Also, replace your o2 sensor. They get old and you start running rich, which kills gas mileage and your cat.

I'm running 31x10.50's and getting 17 mpg with 4.10 gears...FWIW. I don't run hard and drive a mix of 80% two-lane country roads and city.

Ive got 3.73's and balding 265/75/16's and still get about 19 freeway and 15 city although I havent fixed speedo gear yet but I still pull 350 miles a tank -about 15% is how I figured out what my mpg is around .

'92 4.0l 5 spd on 31" all terrains, 3" of lift with stock 3.07 gears. Mostly highway i get about 17.5, if the average air temp is 80* for the week or so, I'll get close to 18.5-19mpg.

In the winter it drops down to about 15-17mpg regardless of road mix.

I've been wondering the same thing about mine.

31's on 3.55 gears with an auto. 01' I6.

Speaking of plugged cats. I wonder if that's nto part of the underhood heat problem too. Don't the 99-01 XJ's have cats built int he header too? I've never really looked...

Mike919: you don't state trans type, but if you have an auto, probably 3.55 gears, standard:3.07

When you figure mileage, are you using odometer reading/fuel put in tank? Do you have a GPS handy(or a phone app)? If so, try using the mileage from the GPS instead of the odometer. Once you figure out just how far you're actually traveling, pay attention to the engines power band. Shoot for a cruise RPM around 2K. This may mean staying in 4th(Std) or 3rd(Auto) on the highway.

A full tune-up, air filter, check cat, O2 sensors, cracked exhaust manifold, a lot can go wrong and cause crappy mileage.

My mpg went down when I regeared to 4.56.
I was getting 19 mpg with 32's and 3.55's at 65 mph.
Now its around 15 mpg at 65. The motor spins too fast.

I tried to be as scientific as possible with the process of calculating these numbers, but failed to link the, seemingly obvious, fact that the odometer is tied to the same gear that the speedometer is...

When I came up with these numbers I set the cruise control to 2k and hit the road for about 130 miles. I filled up before I left and hit reset on the odometer. When I arrived at my destination I immediately filled up again, took a reading from the odometer, and used the fuel volume from the pump...

By not changing the speedometer gear, I've clearly skewed my odometer, which makes perfect sense. The speedometer is about 10 MPH lower, at 70 MPH, than an a speed measurement taken from the GPS app on my phone, therefore the odometer is lower, resulting in a lower MPG...

I honestly have no idea what is in the stock diff, nor do I really care to be honest. All I know is that it's a D35. Re-gearing is definitely in my near future. First things first, I've got to find the funds to buy an SYE, 8.8, etc...

Thanks for the sanity check! I'll report back once I find a new tool - GPS mileage logger!

See now im runnin 31x10.5 stock gear. 2 or 3 inch lift. run the freeway alot, well about 50 % of the time. and im getting right at 13.5 MPG. I have changed the cat with a 100 cherry bomb one and replaced my o2 sensor, put in several fuel injector cleaner. Done everything i feel and im still getting that.. Any ideas? I have a 93 cherokee so there is only that one o2 sensor right?

srmitchell said:

My mpg went down when I regeared to 4.56.
I was getting 19 mpg with 32's and 3.55's at 65 mph.
Now its around 15 mpg at 65. The motor spins too fast.

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That is interesting. In fact, it is down right contrary to the popular wisdom/saying here on .

I too experiance the same phenomenon when I was running 30" tires and 3.55 gears. Shifting the AW4 to 3rd to go up long hills would kill my mileage every time. My XJ just is not fuel efficient at RPM's much above 3,000.

Yea, the 4.0L in line engine love to spin fast. The 4.0L in my XJ hates to spin fast. :dunno:

mike919. Well, if you're "10mph at 70" is accurate, the good news is you don't have to re-run your mileage test. If your numbers were 10 mpg, your actual mileage is about 11.5 mpg (about 17% off) still not stellar, but hey,..

techno1154 said:

..,I too experiencing the same phenomenon when I was running 30" tires and 3.55 gears. Shifting the AW4 to 3rd to go up long hills would kill my mileage every time. My XJ just is not fuel efficient at RPM's much above 3,000,..

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I gotta ask how fast you were driving. My jeep: 30" tires, 3.73 axle gears, 2750RPM at 65 MPH in 3rd gear.

tbburg said:

mike919. Well, if you're "10mph at 70" is accurate, the good news is you don't have to re-run your mileage test. If your numbers were 10 mpg, your actual mileage is about 11.5 mpg (about 17% off) still not stellar, but hey,..

I gotta ask how fast you were driving. My jeep: 30" tires, 3.73 axle gears, 2750RPM at 65 MPH in 3rd gear.

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I usually set the cruise control between 65 and 70 MPH depending.... With about 300 lbs of cargo in the rear and just me in the XJ the AW4 will hunt for the correct when climbing hills. To preventing the gear hunting I manually shift to 3rd. While that take care of the gearing problem, I kiss my OK mileage good bye.

I could see how 3.73 gears would be perfect with the AW4 and 30" tires. I regear to 4.10 and still using the 30" tires. Now hills do not matter. The XJ run over them as if they are not there so much so that I think 31" tires or maybe even 32" would be better suited to 95% highway driving. The next set of tires will be 31". I don't think I will go 32".

techno1154 said:

That is interesting. In fact, it is down right contrary to the popular wisdom/saying here on .

I too experiance the same phenomenon when I was running 30" tires and 3.55 gears. Shifting the AW4 to 3rd to go up long hills would kill my mileage every time. My XJ just is not fuel efficient at RPM's much above 3,000.

Yea, the 4.0L in line engine love to spin fast. The 4.0L in my XJ hates to spin fast. :dunno:

Click to expand...

Yup. I lost mpg on the freeway, regearing.
I used to rev at around 1900 rpms at 65, now its dead on at 2500 rpm.
4:56, 32's. AW4
Things I've done to gain it back (with little success)

-Cold air intake (its a cowl snorkel)
-Free flowing exhaust
-4 hole injectors from precision auto injectors (same flow rate, better pattern)

-Religious throttle body, and intake air temp sensor cleaning
-Only a 3.5 inch lift (it's shorter than a stock tj with a hardtop- Seriously)
-32 inch all terrains aired to 35 psi
-Dual electric fans which don't run on the freeway

4.56 gears, 32" tires,2500 RPM and 65 MPH,... it look like you engine is running about 200 RPM higher that it should be. Take a second look at ever thing. Some thing do not seem right to me.

With 4.10 gears and 30" tires My XJ do 65 MPH at 2350 RPM. I would like to see about 100 RPM less at the same speed. I think a 31" tire will do it for me. I should be able to gain about 2 MPG up from 19 (right now) when I do.

1987 XJ (RENIX) 4.0L AW4, 4.5" 33" BFG ATs, 4.56 Gears

When I first installed the lift, I was seeing about 15 MPG. Over the next couple of years the mileage had fallen to about 13 MPG. I did all the popular tune-up stuff, preparing for a trip to Moab. With that drvie from NY, I would have loved a couple of more MPG. The further I got into the hills then mountains, the worse my mileage got. When I got to Denver, they had record heat. I drove over the pass West of Denver mid afternoon. My engine temperature hit 270*F and the ATF showed 330*F. I deduced my overheating engine was because the ATF was heating the water before it returned to the engine. My mileage on that tank had dropped to just over 10 MPG. I figured the poor mileage and hot ATF were related due to the torque converter not locking up.

At Moab I dropped the TCU and checked all the input signals. Everything was fine everytime I checked. I left the meter connected to the TC lockup solenoid output and went for a drive down the highway. That confirmed the solenoid was not being actuated. I could not leave the TCU loose and meter on while I was running trails. I drove home, cleaing the mountains at night to keep the trans cooler.

I found a package of twenty 3mm LEDs with an internal resistor for 12V operation on ebay for $3.00. I made a small panel with an LED for each of these signals:
TC locked
1-2 select
D select
Brake applied
I used straight pins to connect into the back side of the TCU plugs. I took it for a ride. After about 5 or 6 miles, the Brake LED started flashing. The TCU response is to unlock (or prohibit locking of) the torque converter. This signal comes from the white switch at the top of the brake pedal. The circuit is of a strange design. With the brake pedal at rest, this switch is actuated and provides a ground to the TCU inupt when the pedal is NOT pressed, holding the input to the TCU at 0 volts. There is a pull-up resistor at this input, which provides a 12v signal to the TCU when the pedal is pressed.

The cause of my problem was dirt on the contacts inside the white brake switch. This switch also has a vacuum dump valve, so thee is a small hose attached. Once you get the switch out on the bench, it is easily snapped open to access to the contacts. My mileage is now consistantly at 16 MPG.

MoFo,

Very interesting problem....
Definitely a problem other owners could have.
Great troubleshooting....
Thanks for the information.