A tyre size calculator

Over the years I have read and heard loads of 'advice' on what tyre sizes do and don't affect speedo readings, I even did a load of calculations which I published in the Marina club magazine.  I recently found the following calculator on www.carbibles.com and copied it over to share it - Carbibles is a great website, it's worth taking a look, for all sorts of car related data.  This calculator has everything, it contains a JavaScript application below to help you work out what your old and new wheels and tyres will do to your speedo. Select the tyre size you currently have, and then the size you're interested in.  Calculate each tyre size and then click on the click to calculate the difference button. It will show you all the rolling radii, circumferences, percentage differences including speedometer error. 


By the way, I was challenged previously on the assumption that the tyres are round, when in fact tyre is always a little flat at the bottom, it was suggested that the rolling radius in then smaller.  It's not important, as the calculation is to establish the circumference of the tyre and even if the tyre was totally flat the circumference won't have changed.  Imagine a fully inflated tyre laying on its side and run a tape measure round it.  Then deflate the tyre and do the same thing, making sure that the tape is in contact with the tyre all the way round, the measurement will be the same.  What we are trying to calculate ultimately is how many times the wheel/tyre will turn per time unit (hour, minute etc.) to create a given speed (MPH) and then compare two different tyre/wheel units to check the effect on the speedo.



Current wheel/tyre New wheel/tyre
/ R / R
downarrow downarrow
Current RR:mm New RR:mm
Current circumference:mm New circumference:mm
Difference in circumference:mm or %
So when your speedo reads 70mph, you're actually travelling at mph
Tyre Size Comparison

A Speedometer error means an mileometer error too

It stands to reason that if you change the rolling radius of your wheels and tyres, and the speedometer no longer reads correctly, that your mileometer will also gradually become inaccurate. Assume for example that you bought a car and changed the wheels and tyres on day one from 145/80R13 to 185/70R13 - not an uncommon change. From the calculator above, that makes your speedometer under read by 4.8%. Consequently, the mileometer reading will also be out by the same value. So for example, when you get to 10,000 miles of driving (in the real world), your mileometer will actually read 9,520 miles. OK so that's not a huge difference but it is one of the reasons why most car dealers have a disclaimer on their secondhand vehicles telling you that they won't guarantee the displayed mileage. ("Clocking" the mileometer is the other reason).