Im not sure , you may be right,but it sure dont seem like 20 % judging by the ride to work. Until I get a good check against the gps I wont know. I did compare the speedo to the gps on a few rides and it seemed to be about 10%. The 19" tire probably has a taller sidewall than the 21" tire,so that may make up for some of the wheel size. EDIT: Found a conversion calculator online and it appears to only be a 4 % change. I also remember the speedos are off anyway even with the stock wheel,but I believe the Odo is pretty accurate.So 4% seems right.