Ok, I adjusted the tire size in my Evo and now my speedometer matches my portable gps but the Evo reads 2mph less.

 

cant get it perfect...been trying for years..

 

Mine is the opposite. My evo matches gps, the spedo, however, is off different amounts throughout it's range. It's dead on up to around 30 mph, then off by 2 at 50, dead on at 59, off by 3 at 70. I attribute is to plastic parts and springs wearing out over time.