22545-R18 tires are 2.2 inches (56 mm) smaller in diameter than 235/55-R18 tires and the speedometer difference is 7.8%. Tiresize comparison. 235/40 R18 vs 225/45 R18. Calculate. CM INCHES. If your speedometer reads: Real speed is: Size difference between 235/40 R18 and 225/45 R18 tires. In the tire comparison table above, you can 22540-18 225/45-18 Difference; Diameter inches (mm) 25.09 (637.2) 25.97 Inthe tire comparison table above, you can see a detailed calculation of the size difference between 245/40 R18 and 225/45 R18 tire. The calculation results are presented in the form of visual infographics, on which tire with 22545-R18 tires are 2.54 inches (64.6 mm) smaller in diameter than 225/65-R17 tires and the speedometer difference is 8.9%. 22545-R18 tires are 3.13 inches (79.5 mm) smaller in diameter than 235/60-R18 tires and the speedometer difference is 10.8%. 24540-R18 tires are 0.31 inches (8 mm) larger in diameter than 235/40-R18 tires and the speedometer difference is 1.2%. 23545-R18 tires are 2.3 inches (58.5 mm) smaller in diameter than 225/60-R18 tires and the speedometer difference is 8.1%. uuuYq.