09/28/2026
Our good friend Joel Morrow wrote this very good message. Worth a quick read!!
WHY SLOWING DOWN A CLASS 8 TRUCK SAVES FUEL
No combination of gearing, transmission ratios, or engine torque can eliminate the extra energy a truck needs to travel faster.
At highway speeds, a tractor-trailer uses a great deal of energy pushing air out of its way. The faster it goes, the harder that becomes. Aerodynamic drag rises roughly with the square of speed.
For the same truck on level ground in still air, traveling at 72 mph takes about 44% more aerodynamic energy per mile than traveling at 60 mph. Slowing from 72 to 60 mph reduces the energy spent fighting the air by about 31%. That does not mean total fuel consumption drops 31%, because tires, hills, accessories, and other losses still use energy.
Good powertrain specifications absolutely matter. A well-chosen rear axle ratio and transmission can keep the engine operating efficiently and improve fuel economy. But gearing only changes how efficiently the engine produces the power needed at the wheels. It cannot make the aerodynamic drag at 72 mph disappear.
Here’s the simple distinction:
• Better gearing helps the engine do the required work more efficiently.
• Lower speed reduces the amount of work required to travel each mile.
There are practical limits. Slowing too much may put an engine in an inefficient operating range, cause repeated downshifts on hills, or increase the effect of equipment that burns fuel by the hour. The best cruise speed depends on the truck, load, terrain, and schedule.
But for a Class 8 truck running highway miles, there is no magic axle ratio that cancels the physics of speed. Spec the truck well, drive it where the engine operates efficiently, and recognize that a lower highway speed gives it a major opportunity to use less fuel per mile.