Got a question for you guys, I have a 1993 2wd, 302. I put a 3" body lift on it and changed my tire size from 235/75/15 to 31/10.5/15 A/T. My gas mileage is killing me I am avg. around 10 city and maybe 14 hwy. I used to be able to get 17 mixed and almost 18.5 hwy. Gas prices are hurting me. I am thinking either of changign my tires back to 235, or change my gear ratio. I would like to keep my new BFG A/T on the truck, but gas mileage is bad and the truck struggles to accelerate and seems sluggish. I need to do something

What do you think my best option is, change tires or change gears?
How much would it cost, what ratio would be best, would it help mileage?

Thanks a lot, Nathan

 

What ratio is in it now.
Look for the vehicle ID sticker on the door jamb and post the 2-digit code it lists under "Axle"

 

yea i looked for that too and the sticker was ripped off. I dont know how else to check it.

 

Quick and dirty to try if you dont have Limited slip would be to jack up one rear tire, mark the driveshaft, and count how many times it spins with one rotation of the tire. Make sure you block the front tires while the truck is in N.

Do ya mean 33s? Theres less than 3/4" difference between 235-75s and 31x10.5s. Shouldn't matter for mileage or even height.



235's up front, 31's out back.

Adrianspeeder

 

There's a tag on the rear differential housing. It's fastened on by one of the bolts which holds the cover in place. It has 3 lines of code on it. One of them should contain a string like "3 08" or 3L55" Tell us what that says.

 

Same thing happend to my 94 F-150 4x4 when I put on that same tire size. :-( I had to reconfigure the transmission for the correct tire size, but I also found out that if I slowed down to around 65 on the freeway, I could maintain around 16 mpg. Not sure what to do for power, am thinking of putting on higher flow headers with a bigger exhaust pipe.

 

Yea i was hoping that maybe regearing would save my mileage, but i am thinking of putting 235 back on and just keeping for 31" for hunting/winter season.

 

Smaller tires will give the most benefit.
Larger, heavier tires have more rotational inertia, so they take more energy to turn (and slow down).

 

Smaller tires cause the truck to go LESS distance than the odo is measuring, so it APPEARS to cause mileage to increase. Larger tires do the opposite.

But in fact, tire size doesn't have much impact on ACTUAL mileage. You still have to burn the same amount of gas to get the energy to move the truck's weight (and the tires don't account for much of that) a given distance. A little more or less weight or drag on a heavy, boxy truck doesn't change that enough to matter.

So it's no surprise that you think your mileage has dropped after swapping to big tires. If you plan to keep them on the truck for normal driving, read this thread. If you plan to use them only for hunting season & go back to stock tires for daily driving, don't worry about it.