Car & Vehicle

Tyre Size Comparison Calculator

Compare two tyre sizes: the difference in diameter, ride height and what the speedometer will read.

4 inputs Free, no sign-up

Your figures

The size currently fitted, or the one on the door placard.

As printed on the sidewall - 205/55 R16, for example. Width in millimetres, sidewall as a percentage of the width, rim in inches.

Optional. Shows what the odometer would record over this distance on the new size.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

Changing tyre size changes the rolling circumference, and everything that reads from wheel rotation changes with it. The speedometer, the odometer, the trip computer's mileage figure and on a modern car the ABS and stability control all count turns of the wheel.

Enter the two sizes and this page gives the difference in diameter, what it does to ride height, and what the speedometer will show when you are actually doing 60.

How to use this calculator

  1. Enter the size currently fitted.
  2. Enter the size you are considering.
  3. Optionally enter a distance to see the odometer error over it.

The formula

Diameter        = 2 x (Width x Aspect / 100) + Rim x 25.4
Difference      = (New - Old) / Old x 100
Ride height     = (New - Old) / 2
Speedometer     = 60 x Old diameter / New diameter

Ride height changes by half the diameter difference, because only the bottom half of the tyre sits between the axle and the road. A taller tyre turns more slowly for the same road speed, so the speedometer — calibrated for the original — reads low.

SymbolMeaningUnit
Original Size fitted now
Replacement Size being considered
Difference Change in overall diameter %

Worked example

From 205/55 R16 to 215/60 R16:

Original    = 631.9 mm
Replacement = 664.4 mm
Difference  = +32.5 mm, or +5.14%
Ride height = +16.3 mm
At a true 60 km/h the speedometer reads 57.1

Over a true 100 km, the odometer would record 95.1.

Notes and limits

  • This is arithmetic, not approval. Whether a size suits a vehicle depends on clearance at full lock and full suspension travel, the load and speed ratings, and the manufacturer's specification. None of that is in a size.
  • A difference of more than about 3% is generally considered large, because it affects the speedometer, the odometer and anything reading wheel speed.
  • A speedometer reading low is the dangerous direction: you are going faster than it says. Most are calibrated to read slightly high from the factory for exactly that reason.
  • A larger overall diameter also changes effective gearing — slightly better at a cruise, slightly worse pulling away.
  • Check the placard in the driver's door frame for the sizes the manufacturer approves.

What this calculation assumes

  • Dimensions are nominal, calculated from the sizes rather than measured.
  • The speedometer is assumed calibrated exactly for the original size, which is rarely exactly true.
  • This is not a fitment check and does not say whether either size suits any vehicle.

Frequently asked questions

How much can I change tyre size by?

The common guidance is to stay within about 3% of the original overall diameter, but that is a rule of thumb rather than a specification. What your vehicle actually permits is on the door placard and in the handbook.

Will bigger tyres make my speedometer wrong?

Yes, proportionally. A tyre 5% larger in diameter makes the speedometer read about 5% low - so at a true 60 km/h it shows 57. The odometer under-records by the same proportion.

Does changing tyre size affect mileage readings?

It affects the recorded distance, so a trip computer will report a mileage figure that is off by the same percentage. A tank measurement with a corrected distance is the reliable check.

Is a wider tyre always taller?

No. The aspect ratio is a percentage of the width, so a wider tyre with a lower aspect ratio can be the same height or shorter. That is exactly how plus-sizing keeps the overall diameter constant.

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