OK..."for dummies" so to speak...
1.) You have to rotate both tires at the same time (for 1 revolution) while you count the driveshaft.
2.) If you hold 1 tire and turn the other one, you must turn it twice to get the ratio...I seriously HOPE you don't have 6's...
By the way...if you want them:
Tire size is it's diameter (in case you didn't know.
mph x gear ratio x 335 / tire size = rpm
(rpm x tire size) / (mph x 336) = gear ratio