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Tire Size Comparison

How different are two tire sizes?

Enter two tire sizes and see how different they are, and what happens to the speedometer.

One Click Answers · Tire Size Comparison
oneclickanswers.com/tire-size-comparison · Printed
About this tool & how to use it

What it does

Compares two tire sizes: how much wider, taller and bigger across the new one is, and what the speedometer would read at the new size. The maths is not a fitment approval.

How to use it

  1. Type your current size and the new size, for example 245/40R18 and 255/35R19.
  2. Set the speed your speedometer shows, if you want a different one than 60.
  3. Read how much taller or shorter the new size is, and the real speed when the speedometer shows that number.

A speed in the unit your speedometer uses (miles or kilometres per hour).

One click answer

Your answer will appear here.

Fill in the form and your answer appears here as you type.

Assumptions

  • This page does arithmetic about the size. It does not say whether a size fits or is safe for your vehicle: check the vehicle's tire placard or owner's manual, or ask a tire professional.
  • The speedometer line assumes the speedometer was accurate with the current size and that it counts wheel turns. Some vehicles correct for tire size in software, so check how yours works.
  • A tire in use is slightly smaller than the figures here because it carries a load.

Printed from oneclickanswers.com. Results are estimates based on what was entered; see the assumptions above.

How it works

Each size is turned into an overall diameter: the rim diameter times 25.4 mm plus two sidewalls, where a sidewall is the width times the aspect ratio. 245/40R18 is 457.2 + 196 = 653.2 mm. 255/35R19 is 482.6 + 178.5 = 661.1 mm.

The difference is 661.1 - 653.2 = 7.9 mm, which is 1.21% of 653.2. The circumference differs by the same percentage, because circumference is pi times the diameter.

A speedometer counts wheel turns. If it was right with the current size, a size that is 1.21% bigger around covers 1.21% more road per turn, so the real speed is the number shown times 1.0121: 60 becomes about 60.7.

Assumptions

  • This page does arithmetic about the size. It does not say whether a size fits or is safe for your vehicle: check the vehicle's tire placard or owner's manual, or ask a tire professional.
  • The speedometer line assumes the speedometer was accurate with the current size and that it counts wheel turns. Some vehicles correct for tire size in software, so check how yours works.
  • A tire in use is slightly smaller than the figures here because it carries a load.

Example

Input: 245/40R18 to 255/35R19, speedometer showing 60

Result: 255/35R19 is about 1.21% taller across than 245/40R18 (7.9 mm). When the speedometer shows 60, the real speed would be about 60.7.

Questions

Will a different tire size change my speedometer?

If the overall diameter changes, yes. A taller tire turns less often for the same speed, so the speedometer reads low; a shorter tire reads high. The page shows the real speed for the speed you enter.

How is the diameter worked out?

Rim diameter times 25.4 mm, plus two sidewalls, where one sidewall is the width times the aspect ratio. For 245/40R18 that is 457.2 + 196 = 653.2 mm.

Does this say the new size will fit?

No. It compares dimensions only. Fit depends on the vehicle, so check its tire placard and owner's manual or ask a tire professional.

Last reviewed 2026-10-04.

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