Tire Size and Comparison Calculator calculate tire diameter from code and compare speedometer differences for tire sizes. The first number 215 is the tire width in millimeters. The 2nd number 45 is used to calculate the sidewall height or "aspect ratio", by percentage. So, the sidewall height of a 215/45 R 18 tire is 60% of 215 = 192. 245/35-R19 tires are 0.11 inches (2.7 mm) smaller in diameter than 225/50-R17 tires and the speedometer difference is 0.4%. .

245 45 r17 vs 225 50 r17