Gas mileage

[chicofuentes0224]

I've read all the previous posts I could find.

I have an 88 4.0 AW4 with 3.55 gears and I'm running 31x10.5x15 tires.

The truck only gets driven twice a day. Four miles one way and four miles home. There are exactly three traffic lights between the four miles. The truck never sees over 40MPH.

I've replaced all the sensors, injectors, filters, done a full tune up , put the correct speedo gear in, and even replaced the exhaust mani and gasket and down tube.

When I bought the truck it did 10 mpg

Now after everything has been changed I get 12-14MPG

Here's my question.

Is this normal, because if so I'll stop replacing parts? The engine does have 220,000 miles on it.

I've read guys getting anything from 15-21 MPG. What do you guys think?

Also, no wheels are draging due to brake problems.

[offroader461]

here are my MPG roughly:

 

87 MJ 4.0 5spd 33's 4.10's---26 MPG

90 XJ 4.0 Aw4 32's 3.55's---18 MPG

 

I'm very very easy on them both but if i stomp on them I'm sure the numbers are lower but thats roughly what i avg.

[BLHTAZ]

87 MJ 4.0 5spd 33's 4.10's---26 MPG

There must be more to that story. I have never heard of a 4.0L getting that kind of mileage around town. I have heard of a few getting 21-22 on a trip though.

 

Mine gets 17-18 around town. 8 miles to work...8 miles home and a little running around on weekends with 187K on the ticker, 31x10.50 swampers and 3.07 gears.

 

Chico...it should be better, but depending on how hard the 220K was...that may be all it's going to do. Your short trips and low speeds only contribute to poor MPG. Optimum MPG is typically obtained between 55 & 63 mph for most vehicles.

[Pete M]

Your numbers are perfectly normal for a truck that sees such limited city driving. :thumbsup:

[HOrnbrod]

I've read all the previous posts I could find.

I have an 88 4.0 AW4 with 3.55 gears and I'm running 31x10.5x15 tires.

The truck only gets driven twice a day. Four miles one way and four miles home. There are exactly three traffic lights between the four miles. The truck never sees over 40MPH.

I've replaced all the sensors, injectors, filters, done a full tune up , put the correct speedo gear in, and even replaced the exhaust mani and gasket and down tube.

When I bought the truck it did 10 mpg. Now after everything has been changed I get 12-14MPG

Here's my question.

Is this normal, because if so I'll stop replacing parts? The engine does have 220,000 miles on it.

I've read guys getting anything from 15-21 MPG. What do you guys think?

Also, no wheels are draging due to brake problems.

 

Which speedo gear did you put in Chico? The correct one for your setup is 32 tooth. I ran that same setup for awhile and even with the stroker I got 15 minimum in town, and 19-20 highway. When I changed the gears to 4.11 (and changed the speedo gear to 37 tooth) mileage increased across the board 2 MPG. And my odometer is spot on.

 

3.55s w. 31" tires on a tired engine = engine lugging well below the optimal power/economy band = poor mileage. You got to regear mate! :D

[FxRacing282]

partially clogged cat?

also if the motor is not warmed up it uses 1/3 of the gas that is put in the cylinders. it dosent take much to idle. what temp thermostat you got in there? ive heard that fully open at 200-210 is best for fuel efficiency

[SuperWade2]

Pretty much what I see in my MJ and even worse in my TJ (about 8-10mpg max)... I drive 1.5 Miles each way to work and have 6 or 7 stoplights between here and there... So I rarely go above 35 mph since thats the limit and we have a lot of bored cops...So I've pretty much resigned myself to the fact that the low mileage, slow speeds, short trips and start/stop really kill the mileage... I did nearly a full tank in the TJ all Hwy @ 65 mph ish most of the way and got closer to 12-14mpg...

 

Since I split the daily driving between the TJ and the MJ, I go about a month between fill ups in either/both rigs, and I don't expect to put more than 2-2.5K miles on either of them over a full year period...

 

Wade

[Eagle]

Chico, the first question is -- are your mileage figures accurate? The difference between stock tires and 31s is approximately 11% to 12% (depending on which "stock" size you use for the base). If you mentioned that you corrected your speedometer gear, I missed it.

 

Of course, if you add 10% to 14, you're way up there to around 15.4 -- whoopee. However, for a vehicle driven short distances at lower speeds, that's right in the ball park.

[HOrnbrod]

I've replaced all the sensors, injectors, filters, done a full tune up , put the correct speedo gear in, and even replaced the exhaust mani and gasket and down tube.

 

He mentioned it Eagle. :D

[WahooSteeler]

Chicofooowen-tay, I think it is what it is brother. I'm at 212k and haven't done a lick to the engine, but intend to basically do all that you have. Right now it runs a little rough and I'm probably about 12 around town and 15-16 hwy, but 98% of it is around town. I'd like to think that when I get to doing all the sensors, full tuneup, etc. I could see a 25% increase. But as Eagle so succinctly pointed out, in that oh so endearing way that he does LOL, when you're working from low numbers, even high percentage changes still yield relatively low numbers. I dunno, with over 200k on the clock, if I'm able to get 15mpg zipping around town running errands etc., I'd be pretty damn impressed and certainly happy, especially on 20yr old engineering. And if the occasional hwy trip to a motocross race or something yielded me 18-20mpg, holy $4 gas Batman, I'd be super impressed.

 

Now, I just gotta get something off my mind, and I don't mean to be a jerk about it to anyone. So here it goes. I've seen several people claim they get low to mid 20's on both MJs and XJs running 33s and 4.10s or higher gears in some cases. And to be clear, I'm talking about 4x4s w/4.0s, autos and manuals, not 4cyl 2wd's. These rigs in their prime MIGHT have gotten low to mid 20s on a long FLAT road trip using cruise control. But that's with 225/75 tires and alloy rims. There are simple physics at work here. You only put lower gears in because you need that to turn 33s and, more often than not, steel wheels (vs. alloys) just to get the engine's RPMs back in the optimum operating range. More weight = less MPG. FYI, a quick check on tirerack.com and 225/75 Goodyear RT/S's weigh 26lbs and 33/12.50 MTRs weigh 57lbs. Sorry guys, it don't add up :nuts:

[Eagle]

Now, I just gotta get something off my mind, and I don't mean to be a jerk about it to anyone. So here it goes. I've seen several people claim they get low to mid 20's on both MJs and XJs running 33s and 4.10s or higher gears in some cases. And to be clear, I'm talking about 4x4s w/4.0s, autos and manuals, not 4cyl 2wd's. These rigs in their prime MIGHT have gotten low to mid 20s on a long FLAT road trip using cruise control. But that's with 225/75 tires and alloy rims. There are simple physics at work here. You only put lower gears in because you need that to turn 33s and, more often than not, steel wheels (vs. alloys) just to get the engine's RPMs back in the optimum operating range. More weight = less MPG. FYI, a quick check on tirerack.com and 225/75 Goodyear RT/S's weigh 26lbs and 33/12.50 MTRs weigh 57lbs. Sorry guys, it don't add up :nuts:

Yeah, I'm just an endearing kind of guy. It sort of goes along with the olde pharte schtick. Charter member of Curmudgeons-R-Us, ya know ...

 

Decent highway mileage with larger tires isn't impossible. "Great" is unlikely, but decent is possible. When my '88 was lifted I ran it on 31x10.50s with 3.73 gears. The gear choice was actually an accident -- I had bought the gears for the '88 XJ with the intention of running them with 30x9.50s, then I bought the MJ so I threw the gears in that. My brother handed down a half-used set of 31s, so that took care of the tire size selection.

 

The end result was that the final drive was exactly the same as stock tires and 3.54 gears. I ran those tires on steel rims, so they were a bunch heavier than stock tires on alloy rims. The XJ typically gets 21 - 23 MPG highway. (Best ever was 28, but that was a LONG time ago, when it was new.) The MJ with the 31s typically got 19 -21 MPG highway -- with the tailgate up, and that alone could account for one or two miles per gallon difference. 33" tires and 4.10 or 4.56 gearing wouldn't be that far off stock gearing as to overall final drive ratio.

 

I don't think the weight of the tire makes a huge difference in highway cruising. The additional mass takes more to get rolling, but once it's rolling it has momentum. It doesn't take a lot of power to keep it rolling at 65 MPH.

 

Don't diss the 20-year old technology. The Renix system has a knock sensor, so it optimizes spark advance. The HO system does not have a knock sensor, so the only way it avoids ping is to never allow optimum spark advance. That hurts mileage. I've never heard of an HO Cherokee getting as good fuel mileage as the Renix models routinely turn in. My '88 still gets better mileage, with 270,000 miles on it, than my wife's 2000 XJ gets with only 60,000 on the clock.

[chicofuentes0224]

Ok guys, so we're all in basic agreement that due to the short distance in city driving I do we are basically right on the money for MPG in my situation. I'm not complaining because if I want to save on gas I'll just drive the 49cc scooter. I just wanted to make sure the truck is where it's supposed to be at mechanically. Anyway I didn't think the extremely large wing on the back or ground effects were going to add HP like the add said :rotfl2: One of these days I'll get on the turnpike and head out for a couple hours just to see what my highway mileage is. Oh and Hornbrod I can't remember off the top of my head what tooth count the speedo gear is. BLHTAZ got it for me and I'll check the post to let you know what it was.

Thanks for all the replies guys :thumbsup:

[SoloCamo]

Yup, the mileage is right around where it should be for your setup.

 

I drive about 3 miles round trip to work and back (I need to just start biking/walking, but then the jeep wouldn't get much use besides the occasion running around) and I average around 13.5mpg.

[WahooSteeler]

Chico, in reality we've all just tried to impress you with worthless knowledge. There's a little known secret that I'm willing to share with you that will give you up to a 55HP increase and 30% fuel increase. It's called...The Turbonator! So easy to install, you can even do it while wearing your bikini! And it's only $39.95!!!

 

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And she's not just a customer, she's the Swedish Engineer that owns the company! :D

[chicofuentes0224]

Hey that's my ex-girlfriend. I always wondered where she got all that money :D

[WahooSteeler]

Yup, she's loaded. I love the fact that when I lavish her with gifts it's actually with her own money! Like that bikini and belly ring she's wearing that I bought her. Silly Ingrid, she's so book smart but not a lick of common sense. By the way, she likes her eggs sunnyside, not scrambled. :chillin:

[chicofuentes0224]

And I always thought she'd gotten rich from being a porn star :clapping:

[WahooSteeler]

She couldn't break in to the industry because her screen name was too hard to pronounce using the standard formula: your pet name and the street you grew up on. But we've been practicing her scenes and working on her new persona: Dr. Ingrid Comanche, the Swiss Squaw.