Most large trucks use manual transmissions with many more gears than a standard car. Nine-speed and 13-speed transmissions are both commonly found on tractor-trailers. The procedure for shifting gears on a truck is very similar to shifting with a manual transmission in a car but demands a bit more attention. Most truck gearshifts have a low range and high range, allowing two gears to be assigned to each position on the shifter. As with any manual transmission, only practice will eliminate rough or grinding transitions between gears.
Understand that "gear step" is the amount of RPM (revolutions per minute) change between each gear and that it differs from truck to truck -- though it is usually around 400 to 500 RPMs. Learning it will help you know when to shift. To find it, you will need a large stretch of straight empty road.
Bring the truck up to speed in sixth or seventh gear, then step on the accelerator until the RPMs stop climbing.
Shift into neutral, while stepping on the accelerator to keep the RPMs up.
Grab the shift lever and move it close to the next lower gear and then ease off the accelerator. The RPMs will fall and the truck will shift into that gear.
Subtract the RPM reading at the moment the truck shifted gears from the maximum RPM you hit when accelerating in neutral. This is the gear step. You will need to lower your RPMs by a full gear step when you want to downshift into a lower gear.
Find your engine's maximum RPM by starting in a low-enough gear for the truck to start rolling forward without any throttle. Step on the accelerator hard and sustain full throttle until your RPMs stop climbing. This is the maximum RPM allowed by your truck's governor.
Get the engine close to 1,400 RPM. Almost all truck transmissions can comfortably shift at this engine speed.
Press the clutch pedal 3 or 4 inches downwards with your left foot. You only need to press it down far enough for the engine to disengage from the gears; bottoming it out will wear out the clutch and prevent you from shifting.
Move the shift lever to the next gear upward using light pressure. Note the gear range switch next to the shifter. This lets each position on the shifter be attached to two gears. The first gear position when in "low" becomes fifth or sixth when in "high." If you are shifting from one range to another, you will need to move the range selector lever before you step on the clutch. Doing otherwise could damage the transmission.
Reduce pressure on the clutch pedal while keeping acceleration constant. The new gear will engage.
Brake or ease off the accelerator until there is a gap of at least one gear step between your RPMs and the engine's maximum safe RPMs.
Step on the clutch and shift in the same way as you would shift upward. Once again, if you are switching from high to low gear range, you must move the lever before taking the truck out of gear.
Release the clutch gently until the new gear engages. Continue shifting downward every time a gap of one gear step opens up between your RPMs and your engine's maximum.
Watch your RPMs very closely when you are on a slope. Gravity will drag your truck and can get the engine moving too fast or too slow, possibly damaging it.
Don't shift on curves, in intersections, or crossing railroad tracks. These can all result in a loss of control.