Nobody’s laughing all the way to the bank anymore. It costs about 65 cents a mile in fuel alone to get there. Considering all the factors that affect fuel economy – aerodynamics, tire inflation, terrain, idle reduction strategies, etc. – driver performance remains the most significant. A difference of up to 30 percent can exist between the best drivers out there and the worst.

To get the best performance from your engine at the lowest cost, it’s essential to understand the relationship between torque and horsepower. It’s key to good performance and good fuel economy.

A full technical explanation of the difference between torque and horsepower isn’t necessary, but in practical terms, horsepower is what keeps you running at high speed, while torque is what does the pulling at low speed.

All modern heavy-duty diesels achieve their rated horsepower at the mid range of the rpm band (typically somewhere above 1,600-1,900 rpm). They produce high torque at relatively low rpm (typically between 1,100 and 1,400 rpm). Fuel consumption is lowest at lower rpm. That alone should tell you the engine is most economical in the lower end of its rpm range. But there’s more to it than that (See attached power maps).
 

Today’s heavy diesels – all makes and models – have a very narrow optimum cruise range. Mating transmissions, drive axles, and tire size to meet the engine’s “sweet spot” is an engineering exercise in itself, but it’s critical to good performance and fuel economy.  An ideal highway gearing set up will have the truck cruising at the selected road speed, with the engine running 100-200 rpm above peak torque (depending on the GVW), and a couple of hundred rpm below peak horsepower.

On this Cat C15, the best cruise speed (sweet spot)
is between 1,350 and 1,450 rpm.

The higher horsepower above cruise speed gives you power to pass, the high torque below cruise speed gives you power to pull up a hill. 

In the real world, as you begin climbing a hill, rpm will begin to drop out of the sweet spot into the higher torque range.  From the driver’s seat, it feels like there’s additional power under foot as the engine drifts down the rpm band and into the higher torque range in a pull. 

So, why do we figure all this stuff is worth knowing? It’s useful to know where your “power” is while cruising and when deciding to shift gears.

Here’s a completely hypothetical example. Although the numbers may vary across the brands and models, it’s relative to all today’s engines.

Your optimum engine cruise speed is 1,400 rpm, and your truck is geared to run at 65 mph at that speed. You decide to pass another truck so you put your foot into it and run it up to 70 mph. Your engine speed is now 1,600 rpm. You’re on the high side of the fuel consumption curve, and your mpg might have just dropped from 7.0 at cruise speed to 6.0 at passing speed.

Here’s another situation. You’re approaching a hill at cruise speed. The proper procedure is to let the torque work for you. You’ll burn more fuel speeding up to take a run at the hill, and that will deliver no real advantage in terms of getting to the top faster or with fewer gear changes.
 

As your revs drop from 1,400 at cruise, you get into the peak torque range of the engine where the pulling power is best. You’ve now got a 300-rpm band where the engine is running at peak torque, but not burning any more fuel. You’ll lose a bit of road speed, but the torque will keep pulling. If the hill is steep and you need to drop a gear, shift well before your torque bottoms out at 1,100 rpm. Aim to downshift back into the high side of the peak torque range – between 1,300 and 1,400 rpm.

If you shift too early, you’ll get back into gear at maybe 1,600 or 1,700 rpm – on the horsepower side of the power curve. There’s not much pull there, so you’ll waste fuel and accomplish little until the engine drifts back to 1,400 and below.

On this Cummins ISX, you wouldn’t want to drop the
revs much below 1,150 before upshifting. Fuel
economy would suffer above 1,700 rpm.

As you approach the crest of the hill, you’re at 1,200 rpm but the revs are still dropping. Do you drop another gear or just let the engine pull?

It’s safe to drop a little below peak torque for brief periods. There’s still torque there, just not as much. As you approach the top of a hill, it flattens, so you may be able to keep pulling below peak torque on a shallower grade. Most importantly, you’re still on the low side of the fuel consumption curve. Let the revs drift down, and ease over the top by backing off the pedal as the engine speed begins to increase.  Upshift as soon as you can, and let gravity help you back up to road speed as you start down the other side of the hill – no throttle applied.

“There’s no good reason why anyone should ever pull a hill using anything but torque,” says Dave McKenna, powertrain marketing manager with Mack Trucks.

“Pulling at anything above 1,200 or 1,300 rpm, all you’re doing is turning very expensive diesel fuel into noise.”