http://www.whfolk.co.uk/REOCHUMBER/home.htm
Interesting Excel sheets: http://www.whfolk.co.uk/REOCHUMBER/home.htm#links
Remember to figure out the diameter of your tyre (not your rim!) for correct calculation of speed. For instance, 3.50*19 Avon SMII is 26.9", 4.00*19 is 27.6". It all helps