"Dave Plowman (News)" wrote in message
...
In article ,
Paul wrote:
Thanks Dave, you set me thinking. The last time I changed some
wheels on a car, I went from 13" to 15" and it was the first time
since I had owned the car that the speedo reading tallied with my
satnav, previously the car was always showing about 4mph faster.

As my Megane speedo shows a similar difference, can I expect the
same result?

Be easy enough to work out if you can find out the rolling radius of the
various wheel tyre combinations.

even if the speedo reads correctly after fitting different wheels the
mileometer won't read right.