Tire Compare

The speedometer does not measure speed. It counts wheel (or gearbox output) revolutions and assumes each one moves the car forward by the original tire's circumference. Fit a tire with a different diameter and that assumption is off by the ratio of the two diameters.

with each diameter found from the sidewall code:

What else changes

• Ride height moves by half the diameter difference: $\Delta h = (D_{\text{new}} - D_{\text{old}})/2$.
• Gearing gets taller with a bigger tire. Engine rpm at a given true speed changes by $D_{\text{old}}/D_{\text{new}} - 1$.
• Odometer error matches the speedometer error, so fuel-economy figures drift too.

Plus-sizing

To keep the diameter when going up a rim size, drop the aspect ratio and usually add width: 205/55 R16 to 225/45 R17 to 225/40 R18 all stay within about 1 % of each other. A common guideline is to stay within 3 % of the original diameter so that ABS, stability control and speedometer accuracy are not upset.