Compare an original and a new tire size to see the diameter change and the resulting speedometer error.
Your speedometer is calibrated for a specific tire diameter. Fitting a larger or smaller diameter tire changes how far your car actually travels per wheel rotation, which throws off both your speedometer and odometer readings.
Diameter (in) = Rim Size + (2 × (Width × Aspect Ratio ÷ 100) ÷ 25.4)
Going from a 225/55R17 to a 235/50R18 increases the diameter slightly, meaning that when your speedometer reads 60 mph, your true speed will be a little different.
Using the width, aspect ratio, and wheel diameter encoded in the size (like 225/45R17).
A different overall diameter changes distance traveled per wheel rotation.
The tire's sidewall height as a percentage of its width.