Originally Posted by SpeedTriplemark

While on this subject, i have some concerns on wheel/tire size combos. I've done plenty of Plus 1/2 changes before on previous cars and noticed that Audi specs 245/40/18 (stock on my '11). I just purchased some 19" wheels and obviously want to retain same (or as close as possible) overall rolling diameter. I checked specs on various tires from Conti, Bridgestone, Dunlop and all seem to be at 25.8" DIAMETER (+/- .1")
When checking specs on a 245/35/19 tire, diameter is near identical give or take .1 inch, yet Audi specs a 255/35/19 on their stock S4's (larger overall diameter) with 19's.

I can only assume the stock 19-inched S4 is calibrated as such on speedometer FOR that particular car as well as 18-inched cars.

Any feedback from those who have changed wheel sizes?

Originally Posted by togatown

Waiting for my upgraded wheels and tires to be delivered this week. Before I install them, I am going to calibrate my current speedometer reading with a portable GPS and then check it after the new 19" are installed. As you mention, there is very little rolling diameter change, so I would be surprised if it varied more than 1 mph if at all, but we'll see.