The way the numbers are determined is by dividing the number of teeth on the ring gear by the number of teeth on the pinion, but that's probably not what you wanted to know.
Using 3.73 and 3.42 gearing as an example, the difference is about 10%, which means that the engine would turn 10% faster at a given speed with the 3.73s. And another 10% with 4.10s.
Lower gears (numerically high) are better for towing and put less strain on the drive train. They put more torque to the ground. Tall gears, like 3.42s, can deliver better mileage depending on the usage.
Think of it as levers. A short lever (3.42s) is harder to apply force with than a longer lever (3.73s), but you have to move your hand a slightly longer distance.
Hope that helps, Greg.
