Bad gas mileage

[eastbloc]

I am doing miles/gal

You have a proper speed gear in your transfer case, that gives you correct speed and distance traveled.
You have a constant point at which you fill up, from there you can tell how much fuel you added and how much distance that yielded.

Up to this point i have met 3 Jeepers who have claimed mystical fuel mileages. One was math retarded, other two were using bogus numbers for their math.

As @RMETeeJay said, any stored codes? If you have a misfire that throws a code, for example P304, Jeep will go into fuel consuming mode.

 

[Kyle559]

You have a proper speed gear in your transfer case, that gives you correct speed and distance traveled.
You have a constant point at which you fill up, from there you can tell how much fuel you added and how much distance that yielded.

Up to this point i have met 3 Jeepers who have claimed mystical fuel mileages. One was math retarded, other two were using bogus numbers for their math.

As @RMETeeJay said, any stored codes? If you have a misfire that throws a code, for example P304, Jeep will go into fuel consuming mode.

Im not getting any codes.

 

[DEValken]

stick ya finger inside the tailpipe to look for the soot, (when its cold of course)

Careful, I tried that and got punched in the mouth.

 

[Jerry Bransford]

I’m only running 31s. I still have the original gearing. I don’t have much weight.

It could be that you're running the engine at too low of an rpm for the speed you drive. 3.73 is barely enough for the factory 30's, your engine is on the verge of lugging with 31's... especially if you're running at a slower highway speed. My used TJ came with 4.88 gearing for its 35's which caused the engine to lug. It got 11 mpg, never reaching 12 mpg. I regeared to 5.38 and my highway mileage instantly improved to 14.6.

But to start I'd begin by re-calibrating the speedometer by installing a 33 tooth speedometer gear for those 31" tires. They're traveling more miles per revolution than your odometer is indicating.

 

[Yoda]

I am doing miles/gal

Sort of. Your original tire size is likely 215/75r15. Let's say you drive 150 miles and it takes 15 gallons of gas. 150miles divided by 15 gallons is 10mpg right? Not so much.

Well now that you have a 31" tire, your circumference has increased by 12%. So when the odometer says 150 miles, its actually 150miles plus the 12% increase so in reality you are actually traveling 168 miles. With the same 15 gallons that's 11.2mpg.

Your tires are also 25% wider and are now true AT's vs the factory highway tire, so your rolling resistance is considerably higher.

Your lift kit also decreased your aerodynamics by a considerable amount as well as you increased your frontal area by a decent amount.

Your probably lugging the engine also. If you have 3.07's I'm not surprised at your gas mileage. If you have 3.73's its still not enough. Like Jerry said, you should be running 5.13's with 31's and a 0.69 overdrive. That will put you at 2500RPM at 65mph which is the sweetspot for gas mileage/power.

 

[tr21triton]

Careful, I tried that and got punched in the mouth.

how big of fella was it ?

 

[JMT]

I replaced the o2 sensors and I also recently replaced the spark plugs. I am still only getting 10mpg. Any other ideas of what might be causing this?

It could be your gearing with those larger than stock tires and if you didn’t change the speedo then your mileage and speedometer gauges are wrong and you’re traveling farther than they read. Look what happened when I changed from 33’s to 35’s with 4.56 gears with a proper speedo.

33’s

35’s

 

[Kyle559]

Sort of. Your original tire size is likely 215/75r15. Let's say you drive 150 miles and it takes 15 gallons of gas. 150miles divided by 15 gallons is 10mpg right? Not so much.

Well now that you have a 31" tire, your circumference has increased by 12%. So when the odometer says 150 miles, its actually 150miles plus the 12% increase so in reality you are actually traveling 168 miles. With the same 15 gallons that's 11.2mpg.

Your tires are also 25% wider and are now true AT's vs the factory highway tire, so your rolling resistance is considerably higher.

Your lift kit also decreased your aerodynamics by a considerable amount as well as you increased your frontal area by a decent amount.

Your probably lugging the engine also. If you have 3.07's I'm not surprised at your gas mileage. If you have 3.73's its still not enough. Like Jerry said, you should be running 5.13's with 31's and a 0.69 overdrive. That will put you at 2500RPM at 65mph which is the sweetspot for gas mileage/power.

I have 3.73s

 

[JMT]

I have 3.73s

Yeah, 373’s and a 42rle. Get used to it buddy, or pony up and re-gear. You might try to turn on the OD every time you get behind the wheel and see what happens.

 

[Breto31]

I’ve got a 4.0 6-speed, 4” lift, 33” BFG AT’s, 3.73 gears, and new plugs. I just changed the speedo gear to the correct one about a month ago. Before the Speedo change, I was getting low 11’s. After the Speedo change, I bumped up to mid 12’s (I’ve only ran 1 full tank through it since the Speedo change). I can get around 160-170 miles on a tank before the light comes on.

Most of my driving is around town, and admittedly, I run the rpm’s low. I will be heading out on a 2.5 Highway trip this weekend and plan to check it again.

Mid-12’s is about where I expect it to be with my gearing/setup. 10 seems pretty low to me, but I wouldn’t expect anything drastic to change regardless of what you do to negate it, aside from a regear.

 

[AndyG]

I have gone thru 3 Banks headers on my '98 XJ 4.0L; each of them cracked at the tube junction.

Banks owner here- I’m on number 2 myself.

Every post here has merit- I can’t improve on any of it.

I do think he can improve it, but if it is undergeared, and he likes to get around, he would love it re geared for the performance as well.

You can mod it so it turns he overdrive off as you shift in gear- I’ve yet to drive a 42 that the overdrive was very functional.

 

[pagrey]

2003 and 31" tires 10mpg is way too low. There is a problem. I might look at the catalytic converters or the O2 sensors which I'm sure another member has suggested. Gearing is absolutely not the issue. Injectors, spark plugs, air filter, all the regular stuff is assumed checked and okay. You'll find the issue, just keep at it.

 

[Kyle559]

Could it be my transmission? My auto transmission has trouble getting into gear when it’s cold and seems to be slipping.

 

[Jerry Bransford]

Could it be my transmission? My auto transmission has trouble getting into gear when it’s cold and seems to be slipping.

Doubtful but check its ATF level. Start the engine, keep it in Park, then insure its dipstick is showing its all the way up to the full mark on the dipstick. If it's not bring it up to the full mark with ATF+4. It's better to be slightly overfilled than slightly underfilled.

 

[Brianj5600]

I had rattling pre cats and a random check engine light. I was getting 10ish.

Replaced all O2 sensors, plugs, magnaflow pre cat down pipe and discovered a hairline crack in my rear manifold. Didn’t want to spend the money and I can Tig so I got the suggest rod and “fixed” it. Two months later it cracked again so all back apart and installed a banks header.

Correct Speedo gear matters.

2004 LJ auto with 5:38 gears and 35s I get 13.5 to 14.5 depending on how I drive.

The lower O2 sensors play no role in fuel calculations.

 

[NskLJ]

Could it be my transmission? My auto transmission has trouble getting into gear when it’s cold and seems to be slipping.

The delayed engagement is usually due to the drain back valve leaking allowing fluid to drain out of the torque converter while sitting. Mine does this also. I just start it up and wait 30-60 seconds before I put it in gear this allows the front pump to fill the converter back up.

 

[tr21triton]

I don't know how yall are figuring your mileage. I get about 22 mpp (miles per pound). I feed her a pound of food and she pulls me 22 miles ! and yes mine is a 4 paw drive too !

 

[txbemis69]

my 06 4.0 42rle with 3.73 gears sits on 33” tires and shows 10-12mpg when i check it. i’m not tripping cause i know it needs to be re geared bad and a correct speedo gear after being re geared. overdrive is useless until re geared as well. lack of jeep funds has slowed up progress on the jeep build.

 

[Jerry Bransford]

my 06 4.0 42rle with 3.73 gears sits on 33” tires and shows 10-12mpg when i check it. i’m not tripping cause i know it needs to be re geared bad and a correct speedo gear after being re geared. overdrive is useless until re geared as well. lack of jeep funds has slowed up progress on the jeep build.

That sounds about right for 33" tires, 3.73 gearing, and the 42RLE. Regear to 5.13 asap. My TJ's mpg shot up from sub-12 to 14.6 after I regeared.

 

[Kon]

It could be your gearing with those larger than stock tires and if you didn’t change the speedo then your mileage and speedometer gauges are wrong and you’re traveling farther than they read. Look what happened when I changed from 33’s to 35’s with 4.56 gears with a proper speedo.

33’s
View attachment 304718
35’s
View attachment 304719

What app is this one? Seems interesting.