First, let's define the type of gears we have in our RC cars:

Pinion gear - are small gear that goes on the motor shaft.

Spur gear - are large gears that is on the chassis.

Primary Gear Ratio

To calculate your primary gear ratio:

Spur Gear #of teeth / Pinion Gear # of teeth = Primary Gear Ratio
Hence, for example, if you have 64T spur and a 17T pinion you would get:
64/17 = 3.76

The ratio means that it takes 3.76 revolutions of the pinion to get the spur to turn 1 revolution.

Final Drive Ratio

To calculate this, you would need to know about your internal gear ratio, meaning the gear ratio of your transmission only:

Primary gear ratio x internal gear ratio = Final Drive Ratio
So if your car has an internal ratio of 3.6, your equation would look like this:
(64/17) x 3.6 = 13.55 FDR

This means that the pinion needs to make 13.55 revolutions to get the wheels to turn 1 revolution. The higher the FDR, the more low-end torque you get. A lower FDR gives you more high end RPMS (some people refer to this as more horsepower and top speed). In the case for drifting, a FDR setting between 7.25 and 8.5 should be the ideal ratio since you want that torque to pull you out of a turn. However, there really isn’t any "magic number" for gear ratios, a lot of it is from trial and error, and ratios will vary depending on your chassis setup.