Any reason to run 31's over 235/75/15 tires?

[HenryKrinkle]

I drive an '88 XJ. Mostly daily-driver, some exploring, mild wheeling. I built it to run 31's on a 3" lift with trimmed fenders.

Now that I need new tires (wanting Goodyear Wrangler Duratracs), I'm considering stepping down a size to the approx 29x9.3" vs my 30.5x10.5 size. I have poor acceleration and I think a little extra wear and tear on running gear.

I also plan on more highway driving and I think getting back into the power band of the 4.0 will gain me at least 4-6mpg.

Can anyone think of a reason not to run the smaller tire? How likely is it that 3/4" of extra ground clearance will matter?

P.S.- the smaller tires will almost certainly "look funny" in my wheel wells.

 

[dirty Bakers]

Nope, run what best suites your needs.

 

[VanIsle_Greg]

Agreed, whatever works best for your intended use. I went from the 235x75x16" AT's to 31x10.5x15" and notices a difference in acceleration and mileage. I don't drive far to work, and put on less that 8K per year, so for me, it acceptable. Not to mention some engine mods got me back all the grunt I was missing and even helped a little with mileage.

It did look a little "odd" with the 235's on a 3" lift... get a set of DD new tires, and hunt around the used sites for a decent set of used 31" on wheels for when the mood strikes?

Pics of this XJ?

 

[HenryKrinkle]

I'll get some photos tomorrow. Like I said, it looks right with the 31's, but only gets 16mpg highway and I drive in 3 instead of D under 40mph. I also added an AEM cone filter and high-flow catalytic converter to gain a little power to keep up with traffic on I95.

 

[redthies]

Only reason I can think of is 31s look way cooler! I just dropped from 35s back to 33s on my 80 series LC and while it didn't make a huge difference in power, I think my mileage might actually have gone down with the smaller tire. RPMs at cruise are higher so maybe that's it?

 

[Riptide]

I just did exactly what you are proposing, for pretty much the same reasons. I haven't logged enough miles to form an opinion yet, but my little Samurai needs all the help it can get, so swinging a smaller tire seemed like a good idea. I actually lowered the suspension too, from what the previous owner had slapped on there.

We'll see how it turns out.

 

[HenryKrinkle]

I've read a few different things on tire size. It can be good for highway mpg to be running a slightly larger tire than stock for lower rpm's BUT you need to still be within the power band of the motor. I'm just under the power band right now. It does fine on the highway with the 31's and 3.55 gears, but if I get up to 70mph I can really feel the power kick in. So most of the time I'm cruising just under the sweet spot of the motor, making it work harder= BAD for mpg.

I see a few solutions:
-right gears (4.11's for me) - too expensive right now
-get 31's and go faster on the highway; or just deal with 16mpg 'cause it's not that bad
-get 29's and sacrifice a little rubber for a little power

I did just put new injectors (from a 5.0 Ford) in the old XJ. Noticed a big improvement in idle smoothness and power- even though they are supposed to flow slightly less than stock, my old stock ones must have been clogged up.

I have 2 things I want to do to decide:
-Load up the cargo I want to carry (full water jugs too) and see how much fender clearance is left in back
-Take a run on the interstate and see how the new injectors perform with and without cargo

 

[sargeek]

Get into the power band

I would do what ever it takes to get back into the power band of the engine. On my TJ my tire size slowly crept up from the OEM to 30x9.50x15 to 31x10.50x15. Nothing overly radical, but it slowly took away from the performance on the highway. Off road, and city driving was not affected that much. I had an main bearing in the axle grenade, so I went looking for used differential to replace it with. I went from a 3.73 to a 4.11 and it made the jeep fun to drive again. Performance improved across the board. Acceleration, Fuel Mileage, drivability all improved.

By going with a smaller tire - you will improve the performance of your truck.

 

[CA-RJ]

So look funny, but save some money! I went down a tire size on my 4Runner for the same reasons and though it look a little funny, the mpg gains but a smile on my face.