What's a gear ratio and why should I care?
The gear ratio of your buggy or truck is simply the number of times the motor shaft must turn in order to make the rear wheels turn once. The gear ratio affects the acceleration and top speed of your buggy or truck. The larger the gear ratio is, the quicker the acceleration (assuming sufficient traction) and the slower the top speed.

How should I adjust the gearing?

First of all, don't overheat the motor.

Secondly, make small changes when gearing up. If you make a big change you are likely to cook your motor. Add one tooth to the pinion, two only if the motor was *really* cool the last time. Don't change the spur gear unless you have maxed out the pinion (e.g. 26 teeth for 48 pitch gears).

Thirdly, get somebody to time your laps with a stopwatch. Change the gearing and do it again. Compare the typical laps from each run -- not the fastest laps. Choose the gearing that makes your typical lap the fastest, while not overheating.

Should I gear my buggy the same as my truck?
No, the truck needs a larger ratio because the truck motor must turn a tire with a larger diameter and accelerate a heavier vehicle and a tire with a greater moment of inertia.

What does gear up mean?
When somebody tells you to "gear up" they mean to use a smaller gear ratio, i.e. a larger pinion.