2 replies to this topic

[AliCourtney]

Hi

My 998cc mini originally was fitted with 10" wheels. Recently these have been changed for 12" wheels. The speedo is still calibrated for the original wheel size. I was wondering how many MPH the speedo is likely to be out due to the increased wheel size, currently the speedo shows around 83 at top speed. How close is this likely to be to the speed with 12" wheels.

Thanks,

Ali

[Guess-Works.com]

The rolling diameter of the wheel depends on the tyre, but taking the standard... the speedo is calibrated for 10x145 tyres which have a rolling radius of 1527mm (give or take ), the 145x70/12 tyre has a RR of 1595mm which means you speedo will be out by that same ratio, or (1.04453176162409954158480681074) so when the speedo says 60, you are actually doing 62.7 mph, basically it's reading about 4% slow...

With 165x60/12 tyres it's a little less, about 3% slow...

*note* mini speedo's aint that accurate anyway, so worth checking against a know to be correct speedo, eg a GPS one on a tom-tom, road angle or such like...

Edited by GuessWorks, 29 January 2007 - 12:40 PM.

[AliCourtney]

The rolling diameter of the wheel depends on the tyre, but taking the standard... the speedo is calibrated for 10x145 tyres which have a rolling radius of 1527mm (give or take ), the 145x70/12 tyre has a RR of 1595mm which means you speedo will be out by that same ratio, or (1.04453176162409954158480681074) so when the speedo says 60, you are actually doing 62.7 mph, basically it's reading about 4% slow...

With 165x60/12 tyres it's a little less, about 3% slow...

*note* mini speedo's aint that accurate anyway, so worth checking against a know to be correct speedo, eg a GPS one on a tom-tom, road angle or such like...

Thanks for the help, gives me a rough estimate of the correct reading, i'll check the rolling diameter of the wheel later to give me a close idea.

Thanks,

Ali