Sorry to revive an old thread, I have and 03 42 RLE TJ SPORT with Dana 60 and dana448.9 with 5.38 gears and i get 10 mpg on highway 9ish in city. I use to get 15 with stock axles 3.73 d35d30. All of this is with 33 inch tires Mickey Thompson, is it possible that upgrading to 35inch tires will improve my mpg? Since I am overgeared? Dumb question, i know. For what its worth I have all new IAC, TPS, and MAPS. Is this mpg loss all the added weight? I also have some parts laying in the jeep now because I am moving
What's your MPG with large tires and re-gearing?
- Thread starter jeep_boss
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I doubt installing 35's would improve mpg, how are your O2 sensors? They can and do go bad and often they'll erroneously report a lean air-fuel mixture and the PCM will then cause the mixture to go full-rich trying to correct the erroneously lean mixture report. If you decide to replace them make sure to only install either NTK or NGK sensors, avoid store brands and avoid Bosch O2 sensors which have a bad history in Jeeps.
Very unlikely that installing heavier and larger 35's will increase mpg. You aren't really overgeared with 33's and 5.13. You've got something else going on, maybe a bad O2 sensor or coolant temp sensor.
FWIW the IAC has nothing to do with MPG.
I average about 12.4 with 4.88/6spd/35s.
Even if you fix the leverage problem by having deep gears, you're still fighting the added rotational inertia of 30 pounds x 4 tires and it's a big deal. It takes a lot of energy to get those things spinning, no matter how you slice it.
Even if you fix the leverage problem by having deep gears, you're still fighting the added rotational inertia of 30 pounds x 4 tires and it's a big deal. It takes a lot of energy to get those things spinning, no matter how you slice it.
I have no check engine lights. My scan tool give me about %15 rich fuel trim . To be clear i had these same 33’s with 3.73 gears and got 15 mpg. the same tires with 5.38 not 5.13, 5.38 gears gives me 10 mpg
Fwiw, I had my 4.88 with 32s and went to 35s and while 35s made it not feel overgeared, my mileage still got worse.
I replaced my physical thermostat with a failsafe than again with a normal one a year ago and my scan tool says the computer matches the coolant temp i see on the dash( which would be the same anyway) at 195f it warms up properly and all. Seems the digital matches what the motorad is doing, o2 wise i have no code issues is their something specific I should look for? They have the factory (NTK’s? ) on them. I know this engine is picky with o2 brand and spark plug brand fwiw i pass CalSmog
Also I have no idea what my fuel trim was when i had 15 mpg i got scan tool after i regeared… nothing in the pending codes. I swapped the TPS sensor because of a dead spot it had making it not shift on the highway properly. It helped with shifting
My bad, I misread, however 5.38/33s is still in the same rpm spot as 4.56/33's with a 5 speed. It should be able to do better than 10 mpg. It's like 2700 rpm at 70.
Looks like I will changed these o2 senors out since 10 mpg seems bad even for a tj. And their the orignal sensors and fuel trim is so rich. I’ll be sure to get the NTK.
Alrighty that’s what I wanted to know, looks like these 20 year old o2 sensors are giving me greif.
I've had old O2's not work so well anymore even without throwing a code. So that could be the problem. Hate to tell you to throw parts at it, but there's not much else.
You sure the odometer is 100% accurate and if not, you sure you're doing the math conversions properly?
Do you like the 35s, 6spd and 4.88 combo. I go back and forth between 4.88 and 5.13 for my 6speed and 35s.
Doesn't matter right now though since a lot of gears are out of stock lol.
It's as perfect as I could ask for. Next ratio either direction would feel like a downgrade in some situation (4.56 might be nicer at 75-80 but I'd miss the slow crawl, and 5.13 might crawl better but I would feel overgeared on our 80mph interstates.
You'd be fine with either one but with the 6-speed you'd probably like the 4.88 gearing and resulting rpms better if you are on the highway much. I had 4.88 gears with 35's and 5-speed and it was a great combination... you'd have slightly higher rpms with 4.88 than I did due to your 6-speed so you'd be fine with it.
Yeah my math is correct the odometer is 80 mph when im doing 75 mph and I do my math at empty tank, i own a gas station. If anything im getting WORSE mpg than calculated since i put closer to 19 gallons in a full tank.under 55 mph my speedometer is 1:1. Im 1 teeth off the speedo gearing due to this 33 5.38 setup omix dont make it
I have the 6 speed w/35s and I'm glad I went to 5.13s. I run highway, mountains, and everything in-between.
I've seen it go both ways between 4.88/5.13 with NSG and 35's. I believe @pc1p said he would do his 4.88 if he did it over again. He reused his 5.13 axles from his parted out 5 speed TJ which is how he ended up at 5.13. I've seen a lot of love for both setups and not much hate for either.
The 5 speed is very popular with 4.88/33 and 5.13/35. Considering those setups, with a 6 speed I would likely do 4.56/33 and 4.88/35 with the NSG myself since the overdrive is about 1 gear ratio step lower. Mainly for the 3000-3200 rpm cruise for 75-80 as opposed to 3200-3400 for 75-80. Not that I think either would be a bad choice, just some slight pros and cons. Probably also depends on how fast you drive and where you drive.
I chose 4.88 for 33's with the 5 speed myself. I could have done 5.13 but I just don't need that much gear and at a certain point there is a diminishing return in noise, vibration, mpg, etc.
The 5 speed is very popular with 4.88/33 and 5.13/35. Considering those setups, with a 6 speed I would likely do 4.56/33 and 4.88/35 with the NSG myself since the overdrive is about 1 gear ratio step lower. Mainly for the 3000-3200 rpm cruise for 75-80 as opposed to 3200-3400 for 75-80. Not that I think either would be a bad choice, just some slight pros and cons. Probably also depends on how fast you drive and where you drive.
I chose 4.88 for 33's with the 5 speed myself. I could have done 5.13 but I just don't need that much gear and at a certain point there is a diminishing return in noise, vibration, mpg, etc.