Currently my 2003 is only getting 10 mpg. It is a 4.0 auto with a 3 inch lift and 31 inch tires. Are there any common reasons why I’d be getting bad gas mileage?
Bad gas mileage
- Thread starter Kyle559
- Start date
10 mpg isn't normal, even my TJ with 35's and a 4" SL + 1" body lift gets 14.6 mpg on the highway. I'd suspect a bad O2 sensor, especially since they do eventually go bad. The O2 sensors set the air/fuel ratio and if one goes bad and erroneously reports a lean fuel condition it'll cause the PCM to set it to full rich which will wreck the mpg as well as reduce power.
One more thing that can affect mpg is the rpms being too low which can cause the engine to lug on the highway. Regearing my TJ to 5.38 for its 35's boosted its mpg from sub-12 to 14.6. With your 31's I doubt it's lugging on the highway at normal highway speeds but is your driving mostly around town which will reduce mpg, or do you tend to drive slowly on the highway which would cause lower rpms/lugging?
One more thing that can affect mpg is the rpms being too low which can cause the engine to lug on the highway. Regearing my TJ to 5.38 for its 35's boosted its mpg from sub-12 to 14.6. With your 31's I doubt it's lugging on the highway at normal highway speeds but is your driving mostly around town which will reduce mpg, or do you tend to drive slowly on the highway which would cause lower rpms/lugging?
My driving is about half around town and half highway and I don’t drive very slow on the highways. I don’t notice low rpm’s.
Keep in mind what rpms are very (too) low on a Wrangler are normal rpms for a car. If you're in the very-low or even sub 2000 rpm range on the highway that's too low for a Wrangler running Jeep-size tires. My Genesis and my wife's Lexus are happy on the highway and get great mpg with sub 2000 rpms at 70 mph but my Wrangler definitely would not be.
Vtx531
TJ Addict
How are you checking mpg? That is a massively abnormal figure for mpg that I think there would be something extremely obvious if it was really that bad. flat tires, smoking brakes, gas pouring out of the tail pipe.
Worn spark plugs, bad O2 sensor, filthy air filter and "heavy foot" driving style really kill the MPGs. My XJ got 8 MPG city when I bought it. Plugs were worn flat, air filter was clogged and airbox full of acorns. The upstream O2 sensor was bad too. It got 15-16 after I fixed those issues.
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Don't forget tire pressure, old sparks, and maybe a wrong speedogear reporting incorrect mileage.
Also.. consider adding a front spoiler.
Jkng
Also.. consider adding a front spoiler.
Jkng
10 does seem low for a vehicle that doesn't have obvious issues
However;
However;
I get 12-13 mpg consistently. One thing you need to take into consideration is the tire type.
Mud tires have more rolling resistance than an A/T or H/T.
33x12.5x15. They're falken wildpeak at3w
I'm on falken wildpeak at3w right now. My father runs the same tires and seems to get 2/3 mpg better. Maybe my foot is just heavier.
Could be the heavy foot. I'm a little heavy on the skinny pedal too. Gear ratio also plays a factor like Jerry mentioned.
If you have 3.07 gears and your dad has 3.73's that could cause a difference between your jeeps.
You're undergeared, ESPECIALLY that you have the 42RLE automatic whose .69 Overdrive ratio drops the engine rpms by 31% once you're at cruising speed. With stock axle gears that means your engine is lugging which can really affect/decrease your mpg.
When I regeared from 4.88 to 5.38 for my 35" tires and 42RLE transmission my mpg went from sub-12 to 14.6 mpg. For your 33's I'd regear to 5.13.
Did the poor gas mileage happen recently or has it been steadily getting worse ?
You need to start checking the problems others have posted above.....
You need to start checking the problems others have posted above.....
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.
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.
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I have gone thru 3 Banks headers on my '98 XJ 4.0L; each of them cracked at the tube junction.
Sorry to hear that, seems like a pita. That's a different header style. XJs and early TJ's have a 6 into one vs the later TJ/LJ 6 into 2. I added a flat stock brace to a un-used bolt hole on the PS of the motor by the oil pan. I am hopeful this will help prevent cracking the header. From what I have read the XJ and early TJ header of choice has the small accordion flex sections and its still recommended to brace the PS exhaust to the motor.
Not saying mine wont end up cracking, it may, its only been on for around 8 months.
What header ended up working for you?
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