Originally Posted by dolface
the ideal configuration wrt to tire wear is to have the tooth-count on your cog and chainring be prime, relative to each other.
Relatively prime numbers are not necessarily prime numbers themselves. In order for numbers to be relatively prime, the share no factors other than 1. For instance, neither 81 (3^4) and 100 (2^2*5^2) are prime, yet they are relatively prime. This ends your regularly scheduled math lesson.