If you have a non-posi differential, the spider gears double the rear wheel speed when only one wheel is off the ground. To get an accurate ratio estimate with only one wheel jacked up, it's necessary to turn the rear wheel twice and count the driveshaft turns. Another, possibly more accurate method is to make a mark on the tire sidewall, roll the vehicle straight forward one turn of the tire, and count driveshaft revolutions. I prefer to pull the cover plate, count teeth, and get out the calculator!
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!