Jack up the rear so both rear wheels are off the ground. Rotate the tire one full revolution for posi rear and or and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.
What rear gears you want are best based on how you plan on enjoying the car more and what motor you have in it. If you do a lot of long highway cruises a 3.08 or 3.36 would be good choices. If you do more around-town driving, or like more power off the line a lower gear such as a 3.55, 3.73, or even a 4.11 are better.
the trade-offs are that a lower gear (higher numerical number) results in higher cruising rpm's so highway driving is less pleasant. A taller gearing (lower numerical number) giver lower rpm's for highway cruising but is harder to get the car off the line and requires more clutch feathering to get started.
Everything is a trade-off.
As an example, my '65 L76 (327/365hp) has the 30-30 cam which makes a lot of power but the power range is from 3500 on up. Below that the torque is very low. To compensate my car has 4.11 rear gears to help it get off the line better but the trade-off is that during highway cruising at 60mph I'm at almost 4,000rpm. The sidepipes are screaming loud and everything in the interior is vibrating. Not the best highway cruiser car but around town it's a blast to drive.
