Text Size AAA Bookmark and Share

Tractive load formulas


The efficiency (η) from tank to wheels in a typical modern car averages around one-sixth–only half as good as the powertrain can do under ideal conditions—because most of the time it runs under suboptimal conditions. A hybrid powertrain runs at nearly optimal conditions most of the time, reversing most of those losses, and can also recover much (two-thirds in a Prius) of otherwise wasted braking energy. The energy needed to move the car is determined mainly by its weight, secondarily by its aerodynamic drag and its tires' rolling resistance. For example, a 1995 Taurus needs 6.3 kW of force delivered to the wheels, so if its powertrain were perfectly efficient, it'd get about 149 mpg, but its actual powertrain inefficiencies cut this by about sixfold. Importantly, though, lower mass, drag, and rolling resistance can reduce "tractive load"—the power or energy needed to move the car—by severalfold, leveraging roughly sevenfold bigger fuel savings at the tank by avoiding the losses needed to convert and deliver the fuel energy to the wheels.


Ross, Marc. 1997. “Fuel Efficiency and the Physics of Automobiles.” Contemporary Physics. Vol. 38 no 6: 381-394.