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Tyre Size Calculator

With this tyre size calculator you can:

  • Compare two tyres; for example diameter and width;
  • See the difference a new tyre will have on your speedometer reading;
  • See the difference in ground clearance with a different sized tyre;
  • Get some guidance about the effect on your vehicle of the new tyre;
  • See what equivalent sizes there are to your existing and new tyres;
  • See a graphic comparing the two tyre sizes.

You just need to know a few tyre specifications, then hit Calculate. If you’re not sure about tyre specs, then read the Tyre Help.

There is also a calculator to convert tyre sizes to/from imperial and metric and you may also find the Towing Weights Calculator useful.

Enter your current tyre details

Existing Tyre
Tyre WidthTyre Aspect RatioRim Diameter
New Tyre
Tyre WidthTyre Aspect RatioRim Diameter

Vehicle details

Ground clearance now  mm Optional input. Measure this, or use carmaker's specification. Usual legal minimum is 100mm.
Vehicle width across tyres  mm Optional input. Measure this. The width of the car probably won't change as the body should be the widest part of the car.
Speedo reading at GPS 100km/h  km/h Optional input. Measure this. Any GPS app on a phone will work; drive at GPS reading of 100km/h in a straight line on the level for 30 seconds.

Your reference:

Whatever you want to name this calculation, e.g. "My Wicked Wheels". Not used in calculations. Only a-Z, 0-9 and - allowed.

Email: Leave blank unless you want results emailed to you. Email not stored or used for any other purpose.



All figures approximate; consult your tyre manufacturers exact specs e.g. overall diameter.
NOTE: This calculator does not check the tyre sizes entered actually exist for your desired tyres.
Spec Existing tyre
New tyre
Overall diameter (mm) 802 831 29
Overall diameter (in) 31.6 32.7 1.1
Ground clearance (mm) 210 225 -15
Width of vehicle change (mm) 1800 1820 20
Speedo at 100km/h actual 100 97 -3
Sidewall height (mm) 172 200 28
Version 3 - 30/10/2020 - coding & design by Robert Pepper
The following general guidance may be of use; check whether it applies in your specific circumstances:


The measurements here are calculated from the general tyre specs; in practice, actual sizes will vary a little from these measurements, particuarly offroad tyres which have deep tread. Check the exact specs for your selected tyre in your selected size, and in particular:
  • Your new tyre is taller so your speed will read low compared to true speed, Your gearing will now be higher; fewer revs to maintain 100km/h.
  • As your tyre is taller, you should check fitment in spare wheel bays.
  • You are fitting a significantly wider tyre; you may need a wider rim to suit. In most countries tyres cannot protrude from the bodywork, so consider wheel arch flares.
  • You are fitting a smaller rim; ensure this fits your vehicle, especially front brake calliper clearance.
  • Your new tyre is both taller and wider than the existing; this will mean it is heavier, with effects on handling, fuel consumption etc.

Australian road rules

For Australians, the following should be considered with regard to legality but check your specific situation:
  • Your new tyre is more than 15mm taller - check that's legal, usually limit is +15mm for cars, +50mm for 4X4s.
  • Wider tyre may require wider rim; road regulations may say permitted rim width is +/- 25mm from standard.
  • Also check your tyre's speed and load ratings; in most cases the new tyre will need to match or exceed those ratings, except for offroad vehicles which can use N (140km/h) rated tyres.

Existing tyre - nearest options

Diameter change - one aspect size up and down, same rim and tyre width (mm)

Tyre sizeo/dDifference

Rim change - same diameter and width (mm)

Tyre sizeo/dDifference

Width change - same closest to same rim and diameter (mm)

Tyre sizeo/dDifference

New tyre - nearest options

Diameter change - one aspect size up and down, same rim and tyre width (mm)

Tyre sizeo/dDifference

Rim change - same diameter and width (mm)

Tyre sizeo/dDifference

Width change - same closest to same rim and diameter (mm)

Tyre sizeo/dDifference

Will the new tyre fit my vehicle?

There are so many different tyres and vehicles there is no way any online calculator will tell you for sure your new tyre will fit.

The only way to know is to ask others. So here's your 5 steps to enlightenment:

  1. Find a forum that deals with your car. Facebook, Google etc.
  4. Only if the answer does not exist...ask. And be specific with sizes, vehicle model etc.
  5. Accept with good grace the 15 people showing you links you missed to the answer.

L2SFBC Tyre Size Calculation Results
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1 Comment

  • by Jurgen Loos
    Posted 7 January 2023 16:07 0Likes

    Fantastic information

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