For those of you worried about your speedos', the difference between 205/55/15 and 205/50/15 is 3%. ie if your speedo says 100mph you'll be doing 97mph, at 30mph you'll be doing 29.1mph.
It's easy to work out your rolling diameter 3.142 x ( (rim in inches x 25.4) + (2 x tyre width x profile) )
as an example standard 205/55 r15 3.142 x ( (15 x 25.4 ) + (2 x 205 x 0.55 ) ) = 1905.6mm
Checked my speedo on standard wheels against 2 different gps systems and the speedo reads high about 3-4 mph across 0-70mph and gets worse as speed increases, reading 5-6 mph above 100mph, on a private road officer!! exactly the same result on the two E36's I've had as well.





