Hopefully, you set the calibration by actually sitting on the bike, tires inflated to riding pressure, and measured one complete revolution, then followed the instructions for setting the number on the computer. Just going by tire size may not give an accurate reading. Just changed tires on my mtn bike-same "size", but the new tires have a higher sidewall-if I left the old (which was accurate with the old tires) setting on the computer, then it would be off with the new tires. Normally, for mph reading, you measure one complete revolution, then mult. by 25.4 and enter that number in the cyclometer.