| Changing the ratio to something relatively prime, e.g.
| 91/60 or 89/60, would be advantageous for prolonging
| the life of the gearbox.
You miss my nit-pick. It is not the ratio 91/60 (a single number) which
is "relatively prime" - it is the PAIR of numbers 91 and 60 which are
"relatively prime".
"Relatively prime" is a relation between two numbers; it cannot be the
property of a single number (even if that number was calculated as a ratio
of two relatively prime numbers).
Look at: http://mathworld.wolfram.com/RelativelyPrime.html