235/65-R17 tires are 4.06 inches (103 mm) larger in diameter than 225/45-R17 tires and the speedometer difference is 14.0%. 225/65-R17 tires are 0.67 inches (16.9 mm) larger in diameter than 215/70-R16 tires and the speedometer difference is 2.3%. 235/65-R17 tires are 1.02 inches (26 mm) larger in diameter than 215/65-R17 tires and the speedometer difference is 3.5%. 235/60-R17 tires are 0.93 inches (23.5 mm) larger in diameter than 235/55-R17 tires and the speedometer difference is 3.3%. 225/60-R17 tires are 0.47 inches (12 mm) larger in diameter than 215/60-R17 tires and the speedometer difference is 1.7%. 225/55-R19 tires are 0.23 inches (5.8 mm) larger in diameter than 225/65-R17 tires and the speedometer difference is 0.8%. Aug 12, 2014 · Depending on the exact diameter of the new tires, it can be more than shown by tire calculators. I guess it should not impact mpg overall though it might help in town and hurt out of town. I run 225-65-17, one size up as opposed to one size down, and strictly town driving seems to be 1 mpg down vs same or better mixed and on freeways. 235/65-R18 tires are 1.51 inches (38.4 mm) larger in diameter than 225/65-R17 tires and the speedometer difference is 5.0%. Tire Size and Comparison Calculator calculate tire diameter from code and compare speedometer differences for tire sizes. The first number 235 is the tire width in millimeters. The 2nd number 55 is used to calculate the sidewall height or "aspect ratio", by percentage. So, the sidewall height of a 235/55 R 17 tire is 60% of 235 = 192. So, the sidewall height of a 245/70 R 17 tire is 60% of 245 = 192. The 3rd number 17 (generally preceded by an "R") is Rim Diameter in inches. This is the diameter of the rims bead seat base, not the diameter of the outer rim edge. The rim edge, or lip, extends past the tire bead seat to hold the bead and stop the tire slipping off the rim. y0K7sq.