255/40-17 is almost exactly the same diameter as 225/50-16 (stock AP1 size). Speedo won't be any more "off" than it is now.

 

i have both AP1 stock rims with S02 and azenis rt615 255/40/17 on RPF1 i read between 1-2 MPH slower.

brand-----------------overall diameter-------RPM(revs per mile)----------website
bridgestone S02-----------24.8"------------------840------------------tirerack.com
falken rt-615---------------25"--------------------825------------------edgeracing.com

 

Depends on the specific tires being compared, in some cases there will be zero difference, in some the 255/40-17 is *slightly* taller, but even taking the example above, that's a 1.8% difference, not quite 2mph off at 100mph, not quite 1mph off at 50mph.

IMO, even worst case it's not worth trying to "correct" for, particularly given that the stock speedo reads high anyway. A slightly taller tire will give a more correct speed reading.

Practically speaking, the tires are generally going to be close enough in diameter and rolling circumference not to worry about speedo or odo error.

 

 

thanks guys. Tires will be on tuesday hopefully. I'll make sure to post some pictures.