I know this should be in the wheels/tires section, but nobody seems to go there.

I'm planning on changing my wheels on my 2000 XJR out soon to something bigger (width wise). I love the Asteroids, but 8" wide isn't cutting it, and they're sort of heavy. So I'm looking at a few custom options (Forgestar, Forgeline, Work, a few others). I plan on going to a 10" wide wheel on the back (with some lip rolling) and a 9"-9.5" on the front. I want to go slightly higher rolling diameter, so I was thinking of moving up to like a 285-295/35-40/18 on the back and a 265-275/35-40/18 on the front. So my question is how sensitive is the stock transmission programming to changes in rolling diameter? If I went to a 285/40/18 on the back, that would be an increase of about an inch of rolling diameter, which would affect my effective gear ratio by about .1 (basically it'd be like I had 2.95 gears in place of the stock 3.06s as far as what the transmission is seeing the signal as). Would this be enough to adversely affect the operation of the transmission?

 

Well, using 295/35/18 on the rears would be almost exactly the same as the stock 255/40/18.

 

I had considered that, but no one makes a decently priced 295/35/18. I just can't see paying $300+ a tire when I can go down 10 mm in section width and pay less than $200 a tire (for better tires). I'd also like to go up a little in rolling diameter for aesthetic reasons. So the only options for affordable decently wide rear tires in that rolling diameter range with 18" wheels (I definitely don't want to go any bigger than 18") seem to be 285/35/18's which are a little shorter (~25.85" OD) or 285/40/18's (~27" OD). I'd like to go up to a 305/35/18, but I don't think I could fit those under the stock body work without rubbing, even with a good roll. I eventually want to do a very slight pull to the back arches, at which point that may become an option. So my options are to go to a 275/40/18 which is only .66" bigger in diameter, a 285/40/18 which is 1" bigger in diameter, or to a 285/35/18 which is very slightly (.15") shorter than stock. So a 285/40/18 seems to be the best option for width/aesthetics while still remaining in the realm of affordability, but I don't want to drop $400 on tires and then find out the transmission can't handle the change without a TCM tune.

 

Nevermind, I could not brain yesterday, I had the dumb and was convoluting effects that I shouldn't have. Speed signal is unaffected by rolling diameter since the frequency of the wheel speed sensor reading is unaffected, the only effect would be my speedometer reading incorrectly. So no effect on the transmission. But this leads to a new question. Is there an easy way to recalibrate the speedometer for the new tire height?

 

It can be done at the dealer, if the change is within range. Or, learn how to judge speed using your tachometer.

 

A bit late but I have XKR alloys on my XJR X308

9J 285/35/18 and 8J 255/40/18 fit fine, rolling rads as near as the same