How we worked it out
Updated as you change the numbers.
- In 225/45R17: width 225 mm, sidewall 45% of the width, wheel 17 in. Diameter = wheel + 2 × sidewall.
- A bigger diameter covers more ground per turn, so you go faster than the speedometer shows.
Method and assumptions
The sidewall height is the width × aspect ratio. Overall diameter = rim diameter + 2 × sidewall. A common guideline keeps a replacement within about 3% of the original diameter so speedometer, ABS and transmission behave normally. Check clearance and load rating too.
Questions
How much bigger is a 235/40R18 than a 225/45R17?
About 1.7% larger in diameter (25.4 in against 25.0 in): at an indicated 60 mph you would be going about 61.0 mph.
Sources
Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.