[Espeefan]

Mike, you'll want to look at the planetary or spur gear head motors for powering your truck.

Spur gear head motors tend to be more compact, and their length remains the same, as the reduction is changed by swapping gears. Basically the motor has a small gear, which drives a large gear, which drives a small gear, and this repeats till you get the reduction you need.

A planetary gear head grows in length, as the reduction gets greater. They simply add another stage of reduction, one behind the other till you get what you need. A sun gear, driven by the motor, drives the outer planet gears, which spins a carrier, that drives another reduction stage using the same method, and this repeats itself, till you reach the output shaft.

Spur gear heads have an offset output shaft. A planetary will have an inline output shaft. Does it matter? Probably not on a truck. Both work well. If space is tight, go with a spur gear head motor. They are almost always shorter!

It's up to you to decide what reduction you think you need. The better question is, how fast do you want your truck to go, at full throttle? What's the diameter of your tires? How many volts do you intend to run? You can calculate the reduction needed, based off this information.