Would The Prius Get Better Gas Mileage If It Weighed Less? Part1

Q: I have an idea for a story in your magazine. I was thinking you guys could buy a Prius and pull the batteries and anything at all related to the hybrid portion of the car. Obviously this adds up to a lot of weight—dead weight on the interstate where many people do most of their driving. No doubt this car would then get much better highway mileage. If they built it that way, it would be much cheaper and have a much smaller “carbon footprint” due to much less energy being used to build it. As the car is now, it often has to recharge the battery pack on the highway, hurting its real mileage, not to mention the burden of all that dead weight.



A: Your logic is faulty. Here’s why:



1. Overall weight has virtually no effect on steady-state cruising economy. Fuel consumption at expressway speeds on level ground is determined largely by aerodynamic drag and other parasitic drags such as the tire’s rolling resistance.



2. The onboard battery pack is charged, for the most part, when the vehicle slows down, capturing the energy that otherwise would be dissipated as heat in the brakes. It normally doesn’t get charged during steady-state driving or acceleration. (One exception: On some hybrids, notably Toyotas, the battery will be charged to load the engine when the engine is otherwise running to warm up or provide cabin heat. This avoids wasting the fuel needed to keep the engine running when the car isn’t moving. Clever.)



3. Aerodynamic drag goes up with the square of speed, but the power needed to overcome drag goes up with the cube of speed. It takes eight times as much power—or fuel—to go a constant 60 mph as it does to go 30 mph.



That’s why the Prius, the Insight and other hybrids can actually achieve better miles per gallon during moderatespeed urban stop-and-go driving than on long, high-speed freeway trips.