tommyr

I've ordered my GLE with 21" rims. I was playing with tire and rim size's this morning and realized that the rim size changes the number of revolutions per mile. I always thought that larger rims offset with less tire rubber height. did I miscalculate something?

19" rim with 255/50 tires. Rubber height is 255mm * 50%, or 127mm. Total diameter is 19" + 2*127mm/25.4 = 29"

21" rim with 275/45 tires. Rubber height is 275mm * 45% or 124mm. Total diameter is 21" +2*124mm/25.4 = 30.8" (similar result with the 315/40 rear)

Almost a 2" difference, which explains why I felt the larger rim filled out the wheel well better. But how does the car compensate when calculating speed, etc.

GregW / Oregon

You are correct that the 19" package is undersized. I'm not sure why they didn't go with 255/55 (~30.0") instead. Viewing actual tire specs, they run about 29.1" for the 19s, 31.1" for the 20s and 30.7-30.8" for the 21s. So that's a full 2" difference from smallest to largest. I have no idea how or if any speedometer compensation is done.

bigpern23

Years ago, I upgraded from 15-inch stock wheels to 18-inch Cobra R rims on my Mustang. One day, I set the cruise control 5 mph over the posted limit, got pulled over and ticketed for doing like 11 or 12 mph over the limit. You'd have to think they'd calibrate the speedometer for whatever wheels the car leaves the factory wearing. The speedo is digital now, so it would seem like a simple setting adjustment, but I'm just spitballing.

GregW / Oregon

You are correct that with the information being digital it would seem to be simple to calibrate the speedo to the tire size. That may be exactly what they do, I just don't know. My BMW speedo reads a consistent 2 mph high with stock tire size. I haven't checked this one against the roadside radar signs.

tommyr

Exactly. I'm waiting on tirerack to offer winter tire/rim package, hopefully they will have something on a 20" rim.