Speed reading with different tyre sizes - How does Speedo read with non standard tyres?

The tyres on my VW Golf MK2 are different size to the ones that were originally supplied with the car which means the speed reading isn't the same as the standard tyres.

An excellent site called Car Bible has a calculator to show the rolling radius of different tyre sizes which allows calculation of the difference between the speedo reading and the actual vehicle speed.

 You can see that the radius of the 185/55R15 tyres fitted as standard is 1762mm but the 195/50R15 tyres now on the front wheels have a radius of 1737mm. This means that the speedo reading of 70mph is an actual speed of 69.0mph. Obviously this is very close and not visible on the speedo.

However the rear tyres are 195/45R15 which have a radius of 1678mm.This is a bigger difference and at 70mph shown on the speedo the car would actually be travelling at 67mph. Obviously this means that you are still safely within the speed limit but using larger tyres could mean the speedo reads low and at 70mph you could be actually breaking the speed limit.



Speed reading with different tyre sizes - How does Speedo read with non standard tyres?