[Wisha Haddan H3]

Another way to think of gearing is as a force multiplier.

Your engine puts out a certain amount of horsepower and torque at a given rpm. Your choice of gear multiplies your engine torque by your gear ratio. Low gears trade distance for torque. High gears trade torque for distance.

Gear ratios (automatic transmission)

1st - 3.06:1
2nd - 1.63:1
3rd - 1:1
4th - 0.70:1
Reverse - 2.29:1

Here's a simplified example of a 2006 H3 with the automatic transmission accelerating from a stop light.

When you pull out from a stop, you start in 1st gear. 1st gear multiplies your engine torque by 3.06. It's a slow gear (can't go very fast), but the extra torque gets the truck moving.

As you pass the peak torque RPM (220 ft-lbs @ 2800 RPM), your transmission shifts to second gear. 2nd gear multiplies your torque by 1.63. You've traded some of 1st gear's torque for more distance per RPM, which means you can go faster.

Again, you accelerate past the peak torque (2800 RPM) and now shift into 3rd. 3rd gear is 1:1. You're not multiplying the torque anymore, so you've traded 2nd gear's torque advantage for the engine's natural speed. In other words, your engine RPMs pass directly through the drivetrain to the axles.

Now, let's say you get on the highway and you want to cruise at 70 mph. You aren't accelerating any more, so you don't need extra torque. But you want to lower your RPMs and save gas. So your transmission shifts to 4th gear.

4th is an overdrive gear with a 0.70:1 ratio. Instead of multiplying torque, an overdrive gear actually divides the torque, allowing even more distance per engine RPM. This permits even higher speeds, again at the cost of torque. If you come to a hill, your speed will drop. If you want to pass, you won't have the oomph to get it done. You'll have to shift down to 3rd or even 2nd to generate enough torque to do it.