A few bolts and a gasket, plus a lube change- - - -that seems to be the simplest, most accurate way to address the problem. There's probably no need to count teeth- - - -a stamped number on the ring gear will probably give the ring/pinion tooth count- - - - -9-37, for instance, is a 4.11-1 ratio.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!