How this calculator works
- Step 1Width185 mmTread width, sidewall to sidewall.
- Step 2Aspect65%Sidewall height as a share of the width.
- Step 3Rim15 inchesThe wheel the tyre fits.
The overall diameter is the rim plus two sidewalls. The calculator works it out for both sizes and compares them.
Worked example: 185/65 R15 to 195/55 R16
- Diameter
rim × 25.4 + 2 × width × aspect ÷ 100
- 185/65 R1515 × 25.4 + 2 × 185 × 0.65621.5 mm
- 195/55 R1616 × 25.4 + 2 × 195 × 0.55620.9 mm
- Change−0.10%
The bigger wheel is balanced by a lower sidewall, so the overall size hardly changes and the speedometer stays accurate.
When the size grows
- 195/55 R1699.9 km/hDiameter −0.10%.
- 205/55 R16101.7 km/hDiameter +1.67%: you go faster than shown.
A taller tyre makes the car travel further per turn, so the speedometer reads low.
Assumptions and limits
- Nominal sizeFrom the size code, not measured.
- UnloadedWeight squashes the tyre slightly.
- New treadWorn tyres are a little smaller.
- Not fitment adviceCheck clearance and your car’s approved sizes.
Related vehicle tools
A larger tyre changes distance per turn, so it changes speed and mileage readings.
To check fuel economy after a change, use the gas mileage calculator. For speed, distance and time, the speed calculator does the arithmetic.
Common mistakes
Mistake: Reading the 65 as millimetres.
Instead: It is a percentage of the width.
Mistake: Fitting a size only because it is wider.
Instead: Check the diameter change and the manual.
Mistake: Mixing sizes on one axle.
Instead: Keep the same size on both wheels of an axle.
Checking the method
A separate script re-works every golden value in inches and from turns per kilometre.
Read how we test our calculators, or browse more everyday tools.