Traveling faster than the speed limit in a tractor-trailer is dangerous, not to mention illegal. But there's another reason fleets should train their drivers to slow down-cost. Excess road speed and high rpm waste fuel, which impacts the bottom line.

Jim Booth, a recently retired driver trainer for Caterpillar, has some tips for fleets on how to slow down and save money.

First, fleets must realize that torque, not horsepower, enables drivers to pull long grades and hills without touching the gearshift. It's not like the old days when you had your 335-hp. engine cranked up to 388 and you didn't dare pull below 1,700 rpm, Booth points out. Today, you can pull steep grades at 1,200 rpm and not hurt the engine.

One good reason for running engines in their peak torque range is fuel efficiency. If you're pulling a hill at 1,800 rpm under load, you're using 18 to 19 gallons of fuel an hour-no matter what gear you're in. But if you let the engine pull down to 1,200 or 1,300 rpm, it would use only 15 or 16 gallons an hour, says Booth.

You can also save fuel by cutting back on the amount of time you spend idling. Unnecessary idling wastes fuel, and there are no miles to show for it. At normal idle-about 900 to 1,000 rpm-you're using 0.6 to 1.5 gallons of diesel an hour. So if you're pulling off the road for more than five minutes, shut the engine down.

Here are four ways to help minimize idling time:

  • Keep warmups short; long ones waste fuel.
  • Start cool-downs while you're still on the highway to save idle time at truckstops.
  • Reduce the amount of time you have the AC or heater on while you're in a restaurant or sleeping.
  • Turn off the engine when you're waiting to load or unload. You use less fuel when you shut down and restart the engine than when you sit at idle.

Booth recommends that you start out in a gear that lets you release the clutch without applying power to the throttle. Pressing the throttle uses fuel, even if it feels like you're barely touching it.

To achieve maximum fuel economy, use minimum rpm, minimum power, and the fewest shifts necessary when accelerating. Test after test shows you don't get up to speed any faster by "bumping the governor" because you lose time waiting for the rpm to drop between shifts.

When approaching a hill, you can use the slopes to help pick up road speed on the downgrade, he points out. Then as you climb the hill, let the engine lug down as far as peak torque before shifting. Downshifting before reaching peak torque while climbing a hill costs you a half-mile per gallon.

On the highway, the difference between driving 55 mph and 65 mph is about 3ยข a mile, which adds up over the course of a year. The whole idea is to run the truck in the highest gear possible and at a reasonably low rpm to maintain maximum fuel economy at the legal road speed or below.