64DP AND 48DP are different pitches for the pinions and spurs (same as module in metric). To put it simply, it defines how big the teeth are on the gears.
For Off Road we only use 48DP as 64DP are way too fragile, on road cars use either with a preference on 64DP for a smoother and less noisy drivetrain (as well as more sensitive gearing adjustments).
Depending on how many teeth on your spur gear and pinion, as well as the internal ratio (which is car dependant) you can calculate the overall gear ratio with the following formula
ratio = (spur/pinion) * internal ratio
The internal ratio on the ZX5 is 2.5, on the RB5 it's 2.6
As an example the ratio on a ZX5 with a 78 spur and 17 pinion is 11.47.
Now a smaller number will give you more top speed but less punch, and vice versa.
Smaller pinion = higher number = less top speed
Bigger pinion = smaller number = more top speed
For spur gears, it works the opposite. However in buggies we tend to pretty much stick to one or two spur gear sizes and change only the pinion (main reason being that pinion is much easier to change, another one is you lose your slipper setting when you change spur gear).
Overgearing is when you have a gear ratio that is way too small for the motor, and results in losses in top speed as well as punch, and ultimately can lead to overheating and possible damage.
Undergearing is the opposite, but results in a lot of punch and little top speed. Not as bad as overgearing but can still lead to overheating.
As a rule of thumb a good gearing is usually achieved when the car reaches its top speed bout 3/4 down the straight. There are also factors like the ability to take the jumps properly that can lead to wanting to change the gearing in order not to achieve top speed but better punch for the jumps.
Anyway I think I've got it all covered now.
Anymore questions let us know.
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