[135iDCT]

Originally Posted by froop

My understanding is that the smaller the rim, the larger and softer you can go with the sidewall. Increased compliance in the sidewall will help with launching in the 1/4 mile. Also, smaller wheels tend to be lighter and therefore may reduce your unsprung weight.

For the track though, you usually want stiffer sidewalls for improved steering response.

As for the second question, it depends on your tire circumference rather than rim diameter as to whether you will or will not need to recalibrate your speedo. I highly doubt you would need to reprogram the car.

Basically, if you downsize to a 17 but you increase the sidewall height to the equivalent or close to it you shouldn't notice any difference in speed shown and actual speed travelled. But if you downsize and keep the same sidewall height then your tyre circumference will be smaller and thus you will be travelling slower than your actual speedo read out.

Thnx alot very helpful