SPEED: the biggest factor
Moving a big truck down the road requires the engine, drive train and tires to push against several different resistances.
AIR RESISTANCE (aerodynamics)
TIRE ROLLING RESISTANCE
Air resistance is practically nonexistent at very low speeds, but increases rapidly with speed, becoming a major contributor to fuel consumption. In fact, once speeds exceed about 45 mph, air resistance is more important than tire rolling resistance.
Air resistance is a major part of fuel consumption, and that is why truck manufacturers work so hard to improve the aerodynamics of their equipment.
Tire rolling resistance
Tire rolling resistance is the amount of drag created by the tires as the vehicle runs down the highway. Anybody who has rolled a truck tire across the shop knows it takes some effort. But try to do it at 55 miles per hour with several thousand pounds of load on it!
It takes energy to deform the tire, energy that comes from fuel. Some of it comes back when the tire returns to its un-flexed shape, but some is lost as heat.
Part of rolling resistance comes from the flexing and un-flexing of the tire as it rolls into and out of contact with the pavement.
Even though rolling resistance doesn't increase as fast as air resistance with an increase in speed, rolling resistance is present – and a major factor – at much lower speeds.
Just as with air resistance, the actual amount of rolling resistance is influenced by many factors, including load, speed, inflation pressure, tread pattern, amount of tread wear and tire design and construction.
Increasing speed from 55 to 75 mph can increase fuel consumption by 39 percent,
while cutting the effectiveness of fuel-efficient tires by 27 percent.
[Since engine and drivetrain efficiency is approximately 40%, only about 40% of fuel consumed actually moves the vehicle. That portion would be divided approximately as shown above.]
The relative importance of rolling resistance
Because tire rolling resistance is not the only factor involved, an improvement in rolling resistance doesn't produce an equal improvement in fuel economy.
In the real world, if only 10 percent of your revenue comes from hauling groceries, a 10 percent increase in your grocery volume will not produce a 10 percent increase in your overall revenue. (In fact, in that example, you'd get about 10 percent of 10 percent – or about a one-percent improvement.) In most cases, as we'll see, it takes about a three- or four-percent change in rolling resistance to produce a one-percent change in fuel economy.
Speed & travel time
Bridgestone research shows that speed is the largest single factor affecting fuel economy. In tests, vehicles went from about 5.1 miles per gallon at 75 mph to about 7.1 miles per gallon at 55 mph.