Wanted to post to a question and maybe an FYI, i used a speedo and tach from Ryca but have retained my stock wheel set up and belt drive (for now!).  I recently had the opportunity to check the accuracy of the speedo via a sherriff's dept radar gun...and it is dead on.  Since i have not converted to the 18in rear i assumed it would be off.  Does the wheel size not impact speedo accuracy?  If so, it would seem like mine is not calibrated for a larger wheel.  If wheel size is a factor then those with an 18in rear wheel using the ryca speedo might not be seeing actual speed.  Just wanted to throw this out to those who have converted the rear as an FYI.