Help with gearing please

[Paulstar]

Hi Guys.

Right i've got my car set up and its seems to be ok. I've started to look at my gearing extra. At the moment i have a 21t pinion in there.

Looking on the net at gears and pinions there is loads of numbers that i dont know what they mean

Sorry to be such a novice. Could someone give me a rough guide.

What is 48dp or 64dp??
Also changing the spur gear. What difference would this make to the car??
And the same with the pinion?

Sorry again to be such a novice lol

Thanks in advance.

Paul