For those trying to judge the accuracy of the speedometers in their Mini:
http://www.tomsminisite.co.uk/speedo_calculator.php (http://www.tomsminisite.co.uk/speedo_calculator.php)
This site will let you plug in your car's characteristics and see what the different types of Mini speedometers will read. With creative manipulation of the inputs, you can also use it to "reverse engineer" what your final drive and speedometer drive is likely to be.
You could also use it to determine what drive(s) are needed to "tune" the accuracy of your speedometer.
http://www.guess-works.com/Tech/ratio.htm (http://www.guess-works.com/Tech/ratio.htm)
I've played around with this one. Not sure if they give the same answers or not - I didn't enter any test cases to see.