Or rotate the tire one time and count how many times the driveshaft spins. ie: tire once, driveshaft spins 2.75 times then you would be at a 2.73 gear ratio. It's not exact science but it will be close. Just put a white grease pencil mark on the tire and one on the driveshaft. Have someone rotate the tire one complete revolution and count the mark on the driveshaft as it spins.

PS I can spin the tires on my LeMans with 2.56 gears, so being able to lay down some rubber doesn't really mean much when trying to determine your gear ratio...