Skip the calculators. (They are fine, but not necessary and not as accurate as looking at the exact specs)

Look up the tires you currently have on tirerack, look at the specs. Look for the "revs per mile" number.

Then look up the new tire you will be using- compare those two specs. The % difference will be the % change in indicated speed.

it is well established that BMWs tend to indicate higher than actual...so you can go up a few percent in size and still have an 'accurate' speedo.

A

PS (Think I got that right.... )