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31" tires and effect on RPM??

S

steveareno

NAXJA Forum User
Location
Stevensville , MT
31" tires and RPM??

I have a 99XJ, w/3.55 gears, automatic and is a DD. If I put 31x10.50 tires on stock aluminum rims, what can I expect in engine performance, RPM modifications and highway driving. Will it run up hills, or require downshifting? Change in MPG?
I would like to hear from those who have run 31".
Yes, I have lifted the jeep.
Thanks for the feedback,
Steve
 
I have a 99XJ, w/3.55 gears, an automatic and want to put 31x10.50 tires on stock alloy rims. It is a DD with 3.5 lift. I would like to hear from those who have run 31" on the impact to engine performance, RPM, gas milage. Will it run up hills without downshifting?
Thanks for the feedback,
Steve
 
hey Steve,
i have a 00 w/ 4-5 inches of lift and 31/10.5 bfg ats on it... i didn't originally notice anything on my three inch lift, but now with the 4.5 and a roof rack up top, it seems to be shifting late and also doesn't know what gear to go in up hill(constantly downshifting and upshifting) so when climbing the "big" hills in tenn, kentucky and North caro...i just shift it down in to the third gear drop and shes climbs just fine..after that..i also have a minor tranny leak and if i dont keep that full..shes shifts even worse..good luck..
CPL Maier
 
When I first put on 31's I'm sure I noticed a little difference, but it was so minor that I forgot about it after a week. I could still beat other vehicles off the line, spin the tires... etc.

When I put on 32's a few months ago I definitely could tell. I had to run in 3 like Grifter. I eventually regeared to 4.11's. I think the difference to 31's would be insignificant enough to not be too concerned.

Hope this helps.
 
sorry to hijack,,
hey harvo: how do you like them 4.11's i was thinkin of regearing to that also as i have a stock set from a 94 yj d30 laying around...thanks..pm me..back on topic....31's and rpm?
 
If you want to see the numbers from your change, go here http://www.dfw-tx4wd.org/gears.html and plug the numbers/facts in to the windows, they're all drop down windows. You can do a side by side comparison and see what the difference will be. Won't tell you real world "feel" or mpg, but it's a good overview. Make sure you hit the "calculate" button after you put info in the first time, once for each side, or if you change anything to see what the difference would be.
 
i have 31sx10.5 and went from 3.07 to 4.10 and it made a huge difference. I dont know how much of a difference it would make from 3.55 to 4.10.
 
steveareno said:
I have a 99XJ, w/3.55 gears, an automatic and want to put 31x10.50 tires on stock alloy rims. It is a DD with 3.5 lift. I would like to hear from those who have run 31" on the impact to engine performance, RPM, gas milage. Will it run up hills without downshifting?
Thanks for the feedback,
Steve
Click to expand...
The difference from stock tires is approximately 10%. Which means that for any particular speed, your engine will be turning 10% slower. Also, unless you changed your speedo gear, your speedo will be off by about 10%. Actually, the stock speedos usually read slightly fast, so the speedo change will be more like 8% -- meaning your true speed will be about 8% FASTER than what the speedo says.

Your gas mileage will be slightly worse than stock, and it will shift more goiing up hills.
 
Eagle is right on the money. My 98 XJ is on 31x10.5 BFG MTs & the same setup as yours. Using Interstate mile markers, when I drive 100 miles my odometer will read 93.6 miles. If my math is right that's 6.4% difference on my vehicle. But, these tires are getting worn so the difference is less than if they were new.

If I keep my foot out of the throttle (big if), I'll get around 18 mpg highway. I do carry extra weight in skid plates, ARB bull bar, and about 100 lbs of tools. If I put the spare tire up top I'll lose 3-4 mpg. HTH :repair:
 
I, too, have the 3.55s in my 99 with an RE 3" lift and 31s. I didn't really feel so much the engine lag, eventhough I know its there, but my MPG suffered about 2 MPG. The engine speed has dropped below it optimun RPMs so its not running as efficiently, plus the tires are heavier and it takes more effort to get the larger circumference moving. The 4.10s are just a little high for 31s but really put the 4.0L back in the optimun RPMs for 32s. and not too bad for 33s. That is my plan, anyway, to swap out the 3.55s for 4.10s with the 31s and go to 32s or even 33s when the 31s wearout. Now, I am also doing the acetone treatment, so things are looking up in the MPG category.
 
I have 31's and 3.55's and mine feels like a dog. Felt great before that. I guess it's the non-HO engine. ('88) I run 2k at nearly 70mph so that would be good on gas if you were on the interstate and it was flat. Last tank I got 15mpg with an average foot in town.
 
Threads look funny when TWO are made and then get merged!


One thread per subject please.


hinkley
 
WobblesXJ said:
I have 31's and 3.55's and mine feels like a dog. Felt great before that. I guess it's the non-HO engine. ('88) I run 2k at nearly 70mph so that would be good on gas if you were on the interstate and it was flat. Last tank I got 15mpg with an average foot in town.
Click to expand...

Last in-town average with the 31s was also around 15 mpg. But the RPMs on the interstate is around 2200.
 
my XJ has 31"s and 4.10s and a '88 4.0 with 200k on it. @ 76mph, it cranks out at ~2700 rpm (thats as fast as ill go). It gets about 16-18mpg around town, 20+ on interstate. It started with 31"s and 3.07s...then to 3.73s and finally to 4.10s(the way to go for 31"s). Its nice to be able to go from 1st to 3rd when the tranny is real cold.


-J
 
Thanks for the replys. I don't plan on regearing anytime soon. Sorry about the double posting - my mistake and not intentional.
Steve
 
Hi,

I went to 31's on my '95 4.0 manual. At higher altitudes and hilly/rolling highways I found that 5th gear became mostly unuseable. I was also having to downshift to 3rd much more often than before.

I installed the K&N FIPK II and a Fastman TB and this brought the performance back.

Around this time my fuel economy had fallen to a widely variable 14-17MPG but after the mods I get a much more consistent 17-18.5 MPG.

Mark
 
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