Does changing my car's wheel size affect the speedometer?

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a day ago

Q. I want to put 21in alloys on my Porsche Cayenne, replacing the 20in wheels. It occurred to me that I may have to recalibrate the speedometer and odometer to take account of the larger circumference of the tyre. Is this correct and, if so, how do I do it? GF, Renfrew
A. Speedometers work by counting how many times the wheels rotate over a given time. A wheel covers a fixed distance each time it turns (the circumference) so a speedo can work out how much distance it has covered and the speed.
You would think that changing the size of the wheels alters the circumference, so the speedo will be affected, but remember that what we refer to as the wheel is actually the combination of the metal wheel and the tyre. If you are moving to a larger wheel, you simply fit thinner tyres so the overall diameter and circumference stays roughly the same.
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If the overall diameter of the new wheels is within 1 per cent of the diameter of the current ones it will make virtually no difference to your speedo and odometer. You can work this out for yourself if you're reasonably adept at basic arithmetic, although it is slightly complicated by the fact that tyres are one of the few products that mix metric and imperial measurements in their specification — tyre width is in millimetres, wheel size is in inches.
If you want to do the sums, there's an example later. Thankfully, there are easier options. One is to use an online tyre-size calculator; these are available on many tyre suppliers' websites.
Even simpler is to consult your owner's handbook. Without knowing the year and exact model of your Cayenne, we can't give you the right sizes, but if you don't have the handbook, go to manual.porsche.com and enter your vehicle identification number. The site will bring up the official handbook for your car and you can look up Porsche's recommended tyres sizes for different wheels.
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As an example, we picked a 2023, petrol, 6-cylinder model. The right size for a 20in wheel, front axle, is 275/45 ZR20 whereas for the 21in wheel it's 285/40 ZR21. The first number, 275 or 285, is the width of the tyre in mm, the second is the aspect ratio, or the thickness (height) of the tyre as a percentage of the width. The number after the ZR is the wheel diameter in inches.
So, for the smaller 20in wheel the tyre is 123.75mm thick and for the larger, 21in wheel the tyre is thinner, at 114mm. Convert to inches and the total diameter of the 20in fitment is 29.74in, and the 21in wheel plus tyre is 29.98in.
Of course, the important figure is the overall circumference (ie the distance covered for each turn of the wheel) and to get that, multiply the diameter by π (3.142). So, the 20in set-up travels 93.44in (2,373mm) for each turn and the 21in goes for 94.17in (2,392mm). This is a difference of well under 1 per cent and will make no discernible difference to the speedo or odometer.
As a comparison, it's not much more than the difference in circumference between a new tyre and one worn down to 2mm tread depth.
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One last word of caution: on many later models of Cayenne, there is a menu option that enables you to set the wheel size. This seems to give only the tyre-pressure monitoring system the right information about wheel diameter and doesn't affect the speedometer calibration.
So, look in the handbook and buy the tyre size Porsche recommends for your new 21in wheels and you will still have a speedometer and odometer as accurate as with the 20in ones.Tim Shallcross, independent motoring expertPost your motoring questions below or send to [e-mail]