FWIW, It is NOT the difference in circumference. 225/45 has a circumference of 78.5" vs 78.6 for the 255/40. Revs/mile are 808 vs 806. Speedo error of 0.2%. Speedo is calibrated for the 225/50/16. With the 255 the speedo reads 0.7% too slow. A close match, I'd say. Edward