The gear ratio is the relationship between the engine revolution count and the gearbox revolution count.
Typically, most road cars' first gear ratio is quite low, meaning they can pull away from rest in high-load situations. But once the car is moving, the torque required reduces and therefore, the gearing can be used to increase the speed.
The gear ratios are chosen to make sure the engine revs don't drop too far between gears and the engine stays in its power band. The gap between first and second is usually the biggest, since first works hardest. Going up through the gears, the gap will decrease, until you get to top gear, which will typically be overdriven, where the output is faster than the input. In this situation, fourth gear will be a direct connection, where the input and output spin at the same speed.
Typically the following ratios are representative of what you'd find in a road car:
This means that for every 3.25 engine revolutions in first gear, the output shaft makes one turn. However, it also means that for every llb/ft of torque at the flywheel, the road wheels get 3.25Ib/ft, at a given engine speed.
For motorsport use, the ratios need to be spaced closer together, to make sure the engine doesn't fall off the bottom of the power band.Using the road car ratios as a comparison, a typical race gearbox may use the following: