According to Bridgestone, drivers, loads, trucks, trailers and test courses remain constant throughout a single test.
"If it's too windy, or too hot or too cold, or if it rains or snows, the day's testing is canceled," says Guy Walenga. "In the real world, you don't have the luxury of controlling everything, so the numbers are going to be different."
Michelin's Don Baldwin adds that driver habits, truck aerodynamics, tire pressure and engine maintenance are also significant factors in real-world fuel economy.
"A tire with higher rolling resistance will take more energy to move down the road. Choosing tires with lower rolling resistance and maintaining the pressure in those tires will save fuel," he says. "The percentage of the total will depend on how the other factors are controlled. We have calculators that will predict the effect of the tires based on rolling resistance, including how rolling resistance evolves over the life of the tire. This effect is always taking place, even if it is difficult to see because other factors are not being controlled."
Another reason the numbers don't always add up is apples-to-oranges comparisons. A half-worn, non-fuel-efficient tire may actually be almost as fuel-efficient as a brand-new low-rolling-resistance tire. Tires are at their least efficient when the tread is new and deep. As tread wears away, the rolling resistance improves. While one might be tempted to run, say, a three-month comparison between in-service tires and new fuel-efficient tires, the results might not do justice to the fuel-efficient tire.
The best way to evaluate fuel-efficient tires is over their life cycle - including acquisition cost and casing credit, miles-per-thousandth of tire life, and fuel consumption improvements between the baseline vehicle and the test vehicle, Bridgestone advises.
"Bearing in mind that not all SmartWay-approved tires will yield the same performance, you will save more money on fuel with a low-rolling-resistance tire than you will give up in tire life," says Goodyear's Larry Tucker. "When we did those calculations, fuel was still at $2 a gallon."
From the June 2011 Issue of Heavy Duty Trucking