Changing speedo ratio

[Atrus]

So, I have read that there are a "limited" number of combinations that a dealer has the capability of reprogramming a PCM to accept.

Anyone have any idea of what size tires can be programmed into a stock PCM?

Please tell me 245/75/16 - that's what I hope the distant future holds for me!

 

[06Liberty]

would the speedo really be off by that much with 245/75's? They're only a couple inches taller aren't they? I just don't know if it would be worth the trouble is all....

 

[CRD4Liberty]

It can be done, they just need the wheel rotations per mile to recalibrate the computer. Easy to do. 15 minutes tops.

 

[AndySRT]

yup just had mine done at the dealer. 45 dollars. my trans is shifting like stock again because of it.

 

[o8k]

28" tire stock vs 31" thats about 10% off which means for every 100 miles you drive, your really driving 110miles..... at 400miles thats 40 if your fillup mpg says 20.2mpg you really got 22.2mpg!!!! thats a BIG difference, to me anyways....

im running 29.6" tires and am off by a factor of about 6%ish ive driven it and watched mile markers..... im truely off by that much... bonus is that when i hit 100K miles for my warentee, ill really have driven 106K miles te he he, and ill be stickin it to the man on my warentee..... muahahhaha!

tho i think if i get the 245/75's ill go to the dealer and fix odo, if what andysrt says is right, ANY improvement to my ****** shifting would be a good thing..... that would be a pleasnt thing....

 

[Atrus]

Cool, I didn't realize they had that many options.

I know it's not an enormous amount to be off, but I like to have everything functioning properly.

 

[_UnLiMiTeD_]

hmm...maybe i should check into it by my dealer... But isnt 255/65 pretty damn close anyways?

 

[Corwyyn]

hmm...maybe i should check into it by my dealer... But isnt 255/65 pretty damn close anyways?

Depends on what the stock tires were, you can find it listed on the sticker on the rear edge of the drivers door just above the latch. If you had 215/75s then the 255s are 1% larger; if stock size was 225/75s then the 255s are 1% smaller, really not enough to be a bother. Mine came stock with the 215/75s and my 245/75s are 6.5% larger which means at 71mph on the speedo I'm actually doing 75 (verified with my GPS); I normally just put it at 70 on the freeway and hit the cruise, or if I have my GPS with me I use that for my speedometer...

 

[Corwyyn]

Cool, I didn't realize they had that many options.

I know it's not an enormous amount to be off, but I like to have everything functioning properly.

Main thing is if you put on a lot of miles with the larger tires without resetting the computer then your odometer will be off quite a bit - 4 miles for every 100 you put on it with the 245s (vs. your stock 225/75s). I still need to get mine done

 

[jkl]

im 9 miles off with running 265/75/r16's.

checked it with the gps

put the 245/75/r16's today the speedo was right on.

 

[Atrus]

Main thing is if you put on a lot of miles with the larger tires without resetting the computer then your odometer will be off quite a bit - 4 miles for every 100 you put on it with the 245s (vs. your stock 225/75s). I still need to get mine done

That's my concern. I know it's not a big deal, but I am an Auditor - it's my nature to want things correct

FWIW, I have 235/70's stock as I have the off-road package. The difference will be slightly less. I'd be interested to know what Jeep's tolerance is on what's acceptable to sell...we'd probably be amazed that we are right in the correct range for it with minor tire size upgrades. Also, thinking about BFG A/T's - when they are worn, they are a good .75"-1" diameter smaller than new.

Hmmmm, so if I go from 235-70's to 245-75's, it might really not be worth it to change. I'll have to think about that one...

 

[Gris]

'02s suck. Can either choose 215/75/16 or 225/75/16. That's it. According to 3 local dealers, there are no RPM options, just those 2 fixed sizes.