Single trailers

With tandem drive tractors pulling single trailers, tests showed about 43 percent of tire fuel economy was attributable to trailer tires, and about 57 percent to tractor tires.

Weight distribution predicts trailer tires should contribute 42 percent of the fuel economy effect, drive tires 42 percent, and steer tires 16 percent. That's very close to the 43 percent, 39 percent and 18 percent actually found.

Another way of looking at it is that in this configuration, 44 percent of the tires are trailer tires and they contribute about 43 percent to tire fuel economy.

And, for each 3-percent change in rolling resistance, fuel economy changed by about one percent.

"Pups"

When a tractor with a single drive axle was pulling two pups, the trailer tires accounted for 64 percent of the fuel economy effect, with the tractor tires contributing 36 percent.

Again, axle weight distribution predicted very similar results. With pups, it took about a 4-percent change in tire rolling resistance to produce a one percent change in overall fuel economy.

Or, with this configuration, trailer and dolly tires are 67 percent of the tires and contribute 64 percent to tire fuel economy.

Part of the reason for this is that with pups, there are two trailer air gaps, and therefore, air resistance has a bigger effect on fuel economy than it does with a single trailer.

Drive axle effects

In all the configurations tested, the contribution of drive tires to tire fuel economy was a bit less than predicted by weight distribution.

Bridgestone engineers believe the dynamics of drive axles are different from those of essentially free-rolling axles, like steer and trailer axles, and this may account for part of the discrepancy.