4.0L I6 F fuel mileage

[Mikey1989]

I've filled up three times now with my new '89 and am getting about 14 MPG city driving. I've tested the 4x4 but not done any driving with it. So this is all 2H driving.

 

I am looking forward to a road trip to see what the highway MPG looks like.

 

Is 14 MPG city 2H driving par for this truck? Mileage is 130K.

[89 MJ]

That sounds about right, but more information would be nice. 
What trans do you have? Tire size? Gear ratios? What is your driving style?

[87MJTIM]

I was getting 20 +/- on highway when I had 3.07 gears.  I swapped to 3.55 and MPGs fell to 17/18 on highway.  Stop-n-go/city was in the mid-teens.

[Eagle_SX4]

That seems about right. I get about the same in the city. Last week I put about 300 miles on my truck doing about 70 mph on snow tires and got 17.5 MPG. I have gotten as good as 23 by driving the byways (with street tires) and doing no more than 60 mph on a trip before.

[gogmorgo]

Unfortunately yeah, that’ll be roughly ballpark. Basically everything that makes a truck decent off road detracts from fuel economy. Gentle driving can bring the number up some, but you’ll only see so much out of it. Keeping up on maintenance will definitely help. 
 

Fuelly.com is a good resource for seeing what other people are getting, based on real-world numbers. It won’t be a perfect comparison because you won’t know the specifics of their driving, but it’s a good approximation.

 

If you really want to bring the numbers up you can look into hypermiling techniques. The biggest improvements I’ve found are just from sticking to the speed limit and avoiding routes where you’ll be spending a lot of time stopped, moving slowly, or accelerating and decelerating a bunch. But there’s only so much you can do. 

[Htchevyii]

That sounds pretty typical. I got an average if 17 on our last 2000 mile trip, loaded down with 33's. That was everything from 75 MPH interstate to off road trails, but not much stop and go.

[Salvagedcircuit]

I get about 17-18MPG with my '88. There have been claims that MPG can improve if you move the IAT (Intake AIr Temp) sensor from the intake manifold to the air box, but it looks like the results are mixed. You're pushing a brick through the air at 60+mph. Not much you can do with that

[pizzaman09]

One thing worth noting is tire size, I run 235 75 R15 on my truck instead of the factory 225 75 R15, which results in about 2% less miles reported on the odometer vs distance traveled.  This artificially reducing calculated fuel economy.  Tire type also plays a role, I did see a reduction in economy when switching to all terrain tires from light truck tires.

 

14 ish is about right in town with my 90 manual I6 4x4.  Lifetime average over 83 tanks of fuel for me is 16.7 mpg.  Best tank ever was 24.2 mpg hypermiled on the highway.  When I tow a larger trailer around a lot, I tend to get about 12mpg.

[75sv1]

With my MJ, '88 2WD and Manual trans, I calculated 23 mpg highway. I am not sure of the gearing. I gave that axle to another MJ owner, and he thought it was way peppier than his original. Some say the RENIX gets a bit better MPG than the HOs. Also, a manual gets roughly 2 mpg better than an auto. On my XJs, two '98s, and my '00, all autos. The stock green one, with a smooth running 4.0L, 18 to 21 (once) mpg highway. The Azimuth (PJJ, Purple Jesus Jeep), lifted 4.5", about 17-18 mpg with 31's and 4.10's.  With 32's about 16 to 17. The 2000, lifted 2" and 31's was getting me 16-17, with 3.73s. I can't explain the whys. The last trip in it, I changed to a synthetic in the front diff. Did a cut-n-fold or comp cut on the rear quarters, and added window air shields. I went to 18.5 to 19.5 and one tank to 20.5. Then the #6 piston lost its skirt. Towing with the Green one was 14 mpg. I towed some Saab Sonnets. so 2,000 lbs. Spooky looking back in the rear view and having a car on your tail. 

[89 MJ]

I average about 15 with a mix of driving in town and on country roads. My best tank ever was just shy of 20 mpg, running about 75 mph, but it was highway only. No towns, no stoplights, just cruising in 5th gear. I’ve got a 4.0, AX15, 4.10s, and 31s. My tires do seem to be on the harder side, which helps. 

[75sv1]

On my '98 with 31's, the width was 10.5 inches. On my '00 its around 9.5 inches, also 10 ply. I do run those with around 36 psi. Also, both have Ford 8.8s. I give my MPG as Highway only. I'd say 14-15 mpg on country roads and some city driving. I haven't done much driving with Cruise Control with the XJs. On my Ford Contours, I'd get +2 with Cruise. Possibly better. I like to drive about 72 mph. Sort of a tweener. So, I have to get out of cruise control a lot. 

    So, the OP's 14 mpg in the City is fairly good. 

[Dan Schachner]

This is a long post and relates to the Cherokee, but he goes very in depth about improving the mpg of the 4.0

https://www.jeepforum.com/threads/my-goal-is-to-get-25-mpg-from-my-99-xj-with-30-x-9-5-r15-ats.1551782/

[75sv1]

5 hours ago, Dan Schachner said:

This is a long post and relates to the Cherokee, but he goes very in depth about improving the mpg of the 4.0

https://www.jeepforum.com/threads/my-goal-is-to-get-25-mpg-from-my-99-xj-with-30-x-9-5-r15-ats.1551782/

I followed that thread. Even put in a few cents. He did sell that XJ. I tried to buy it. Money just didn't come in time. Anyways a lot of info. There is another thread that claims 27 mpg. That one went with aerodynamics. Mainly, plugging the slats in the grill and then holes or such in the bumper. I have an IRO bumper on my '00. When I did the cut and fold or comp cut, the lines matched better between the bumper and the rear quarters. At some point, I want to put a ZJ grill in front, and also some type of shield in front of the head lights. Sort of like a ZJ. Yeah, a bit weird. Also, see about a flap between the gas tank and the bumper. 

[gogmorgo]

7 hours ago, Dan Schachner said:

This is a long post and relates to the Cherokee, but he goes very in depth about improving the mpg of the 4.0

https://www.jeepforum.com/threads/my-goal-is-to-get-25-mpg-from-my-99-xj-with-30-x-9-5-r15-ats.1551782/

This was a long rambly post. But to sum it up, his recommendations are 

 

1: basic maintenance. Ignition parts in particular, and also good synthetic fluids not just in the engine but also diffs, trans, tcase. And the air filter. Also the recommendation of good battery cables and electrical system, preventing voltage drops and unnecessary loads on the alternator.

2: keep the engine running in its sweet temperature spot. He recommends a 205° thermostat for environments that let passive cooling work, but stock 195 for when it’s hot because the 205 will make fans come on more often. Passive cooling methods like larger radiator, oil and transmission coolers, and hood vents in hot climates. Exhaust wrap to reduce engine bay temps if necessary.

3: aerodynamics. Don’t put anything on the outside of the vehicle, brush guards, winches, roof racks, no spare tires or cargo carriers on the roof. He recommends skid plates for underbelly smoothing. He also states the importance of not lifting and especially not levelling a vehicle, leaving a rake. Also larger tires are a concern, wider is a slightly bigger concern than taller but both hurt. Aero is obviously more of a concern on the highway, but around town keeping the extra weight of accessories off is an advantage too. He also talks about economodder principles here, like blocking off parts the grill, on adding a corrogated plastic belly pan, but adding the caveat that if loss of passive cooling airclow airflow makes active engine cooling work harder it doesn’t benefit you. Which is a good segue into 

4: Don’t load the engine more than necessary. Passive cooling preventing engine fans from coming on. He recommends against additional cooling fans, or anything that will increase mechanical or electrical load. He unplugged the a/c compressor to stop it coming on in defrost, and said it didn’t work anyhow so left it unplugged in summer.

 

5: engine mods. He recommends a high performance ignition system which allows opening up the spark plug gaps a bunch, but also said this lead to running 89 octane over 87. He recommends a 2.25” cat-back exhaust and mandrel bent downpipe on a stock exhaust manifold, or 2.5” but if you have a higher flowing manifold/header to match it. He also went with 4-hole injectors. 
 

6: driver mods: slow down. He claims an 8% increase in mpg doing 60mph instead of 65. It’s one of the more substantial single items on his list and costs $0 to implement.

 

I didn’t look down the rest of the thread to see anything else claimed, but all of the above is in agreement with basic automotive principles. It passes the sniff check. 
 

Something else I’d add is front axle rotating mass is something we take for granted. The factory CAD isn’t ideal, but it does help some with MPG. Locking hubs would be better. I don’t religiously keep track of my own MPG anymore, but I was doing so for a while, around when I swapped from a CAD to a one-piece axle, and while the data is scattered enough I can’t pinpoint an exact when in the data, I do know that all my best MPG numbers were recorded prior to my CAD delete. I went from 17-18mpg averages mostly highway with the occasional 20 to 17-18 with the occasional 18.5. I’ve done other stuff too, like a brush guard and chunky tires, so it’s not just the CAD bringing me down today, but the CAD was the only mod of substance for some time and I haven’t touched 20 since doing it.

[75sv1]

I have been using ceramic coated headers since around '95. That was on my Bricklin. I used some Ceramic coated headers on my '88 Comanche. I did wrap the Brick's headers previously. They rusted up a bit. Hood temps with both felt like 50% reduction. I presently have Thorley's on my '00. 

     I went with a Ironman 4-link on my '01 to 'gain' a belly pan. Not sure how that will work out. Adding a lot of weight elsewhere too.