as for gear ratio, its how many times the engine spins for 1 turn of the rear wheels. With the heavy duty 4 speed (commonly known as the sm420) with the low first gear, its around 7:1, then piped through the rear end which could be 4.5:1. So the first gear means the engine spins 7 revolutions to every one turn of the driveshaft. Then the drive shaft will turn 4.5 turns to every turn of the rear wheel. So you end up with 31.5 turns of the engine for every turn of the rear wheels!
So the engine spins fast, the wheels spin slow, and you have a mountain of power. On the highway you want more speed and the mommentum of the truck helps it keep going. So in 4th gear, the tranny ration is 1:1 or direct, and every turn of the engine is 1 turn of the drive shaft, and that means the engine turns once for every 4.5 turns of the drive shaft (assuming a final drive of 4.57). So that means the truck moves along much quicker, but with much less power.
You'll find that the 4.57 will let the truck go around 50 without pushing it too hard. If you want to go faster you can plug in a different rear gear set of 3.90 or 4.10 which will get you up to around 60. You can also put on really tall tires on the back of the truck and they'll help it move even faster.
My truck had 5.26 gears as it was a 1 ton. I found some 4.10 gears (rear end) and that was a big help to go quicker on the highway.
