22545-R18 tires are 0.35 inches (9 mm) larger in diameter than 215/45-R18 tires and the speedometer difference is 1.4%. 22540-R18 tires are 0.53 inches (13.5 mm) smaller in diameter than 215/45-R18 tires and the speedometer difference is 2.1%. Also the online application calculates the difference in real speed and speedometer readings if the difference between tire 225/50 R18 and tire 235/45 R18 is significant. If you want to compare other tire sizes, then select their sizes in the drop-down fields and click on the green button. 225/45 R18: 215/60 R16: 245/40 R18: 225/55 R17: 22540-R18 tires are 0.89 inches (22.5 mm) smaller in diameter than 225/45-R18 tires and the speedometer difference is 3.4%. 24540-R18 tires are 0.61 inches (15.5 mm) smaller in diameter than 235/45-R18 tires and the speedometer difference is 2.3%. Thedifference in sizes and parameters between the 245/40 R18 tire and 245/35 R18 tire. Detailed calculations and graphics. Tire calculator online . Top comparisons; 225/40 R18: 205/55 R16: 225/45 R17: 235/40 R18: 23545-R18 tires are 0.53 inches (13.5 mm) smaller in diameter than 225/50-R18 tires and the speedometer difference is 2.0%. Diameter width and speedometer differences between 225/45-R18 and 225/45-R18 tires TireSize and Comparison Calculator calculate tire diameter from code and compare speedometer differences for tire sizes. The first number 225 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 225/45 R 18 tire is 60% of 225 = 192. 24540-R18 tires are 1.14 inches (29 mm) smaller in diameter than 225/50-R18 tires and the speedometer difference is 4.3%. Q8xi.