My Xk8 has the 18" standard wheels and I would like to trade up to a larger rim size. Logically this wouldseem to affect the various electronic components that rely on wheel diameter like the speedo, odo and maybe the ABS. Am I right?

If so, do people who upgrade justlive with the issue or can the ECM be reflashed?

 

on your car it wwont matter you cannot program the instrument pack for larger wheels, just dont go nuts with the overall size. that car had an option for 20" BBS with I think was 285/30/20 rear and 255/30/20 front

 

I've been using programs like this for years to calculate overall wheel & tire sizes and determine the revs per minutes and the spedometer difference.. as long as you keep the new diameter within %3 of the old you generally won't even notice the difference. Eric

Overall Tire Size Calculator

 

Spurlee, the ABS won't notice a difference as long as the wheel diameters front and rear are the same. On my truck, I went from 30" tire diameter to 37" tire diameter. ABS works just fine. As for the speedo, well, that was off as you can imagine. But, I corrected that by installing new gears. Not quite that easy with these vehicles.

Like was mentioned, as long as you keep the overall tire diameter close to what you currently have (3% is a good rule), all will be good. At 60 mph, you are only looking at a 1.8 mph error (ie, when you are actually doing 60 mph, the speedo will indicated 58 mph). Not that big of a thing to the car. The computers have a certain amount of give to them due to people changing tire brands, going with different rims, etc.

 

I think the whole purpose is to fill the large wheel wells of the Xk8 better than the 18 and the tires on them do. The 20's do a rea;;y nice job and the tires are smaller in front than the rear. You wont have an issue, and as far as the speedo, if you have a gps you can track the actually speed by satalite vs the speedo

 

This looks like a winner for a new tire size if you decide to go with 20s, it only adds a tenth of a mph to your spedo...