Also, the online application calculates the difference in real speed and speedometer readings if the difference between tire 245/45 R20 and tire 255/40 R20 is significant. Calculate the difference in inches. If you want to compare other tire sizes, then select their sizes in the drop-down fields and click on the green button. 245/45-R18 tires are 0.04 inches (0.9 mm) smaller in diameter than 245/40-R19 tires and the speedometer difference is 0.2%. 245/40-R17 tires are 0.96 inches (24.5 mm) smaller in diameter than 245/45-R17 tires and the speedometer difference is 3.8%. Also, the online application calculates the difference in real speed and speedometer readings if the difference between tire 245/45 R18 and tire 245/40 R18 is significant. Calculate the difference in inches. If you want to compare other tire sizes, then select their sizes in the drop-down fields and click on the green button. 245/40-R20 tires are 0.96 inches (24.5 mm) smaller in diameter than 245/45-R20 tires and the speedometer difference is 3.4%. In the tire comparison table above, you can see a detailed calculation of the size difference between 245/45 R17 and 275/40 R17 tire. The calculation results are presented in the form of visual infographics, on which tire with size 245/45 R17 is on top, and tire with size 275/40 R17 below. Both in the table and in the graph, the tires are In the tire comparison table above, you can see a detailed calculation of the size difference between 245/45 R17 and 255/45 R17 tire. The calculation results are presented in the form of visual infographics, on which tire with size 245/45 R17 is on top, and tire with size 255/45 R17 below. Both in the table and in the graph, the tires are Published by Dustin Babich on 12 June 2023. The main difference between 245/45zr20 and 245/45r20 is the tire type. A ‘z’ rating means the tire has a speed rating of over 149 mph, while ‘r’ indicates a standard performance tire with a maximum speed of 106 mph. Both refer to the same size of tire – 245mm width, 45mm aspect ratio, and a 245/40-R18 tires are 0.41 inches (10.5 mm) larger in diameter than 265/35-R18 tires and the speedometer difference is 1.6%. Better Fuel Economy Factory Ratios Increased Power. vs 275/40-20. 275/45-R20 tires are 1.06 inches (27 mm) larger in diameter than 245/45-R20 tires and the speedometer difference is 3.6%. cvdBQ.