http://www.powerdog.com/tiresize.cgihttp://www.powerdog.com/tiresize.cgi
Handy when converting from 15s to 17s, suprised that the difference with mine is only 1.9%:
Specification
Sidewall
Radius
Diameter
Circumference
Revolutions
Difference
185/55-15
4.0 in
11.5 in
23.0 in
72.3 in
876 / mile
N/A
205/40-17
3.2 in
11.7 in
23.5 in
73.7 in
860 / mile
1.9%