Car geekery part 2: why pulse-and-glide works

Most people think that aggressive driving gives you bad gas mileage. But those who obsess about miles per gallon ("hypermilers") have some counterintuitive techniques. One of them is "pulse and glide", which calls for almost flooring the accelerator. Why does it work?

Gas engines have a throttle that prevents air from freely flowing into the engine (unless you're at maximum acceleration, when the throttle is wide open). The engine needs the correct fuel-to-air ratio, and this is how it's achieved. But when the throttle is holding back the air, the engine is forced to work to pull in the air it needs. The engine is acting like a pump, consuming energy. Holding the throttle wide open minimizes those pumping losses.

This is why big engines get lousy gas mileage: most of the time their throttles are almost completely closed and they are literally sucking air.

The bad news is, you can't really take advantage of this technique with an automatic transmission. In order to make it work, you have to be able to choose what gear you're in, and an automatic takes that choice away from you. Besides, let's face it, you can only floor it when there's nobody in front of you. Hell is other people, isn't it?