As well as the "What kind of car should I buy?" questions, we're suddenly getting a lot of questions about driving techniques to save gas.

Shouldn't be surprising; the average cost of a gallon of gasoline passed $4 this week.

But one of the most confusing areas of gas saving turns out to be speed, epitomized by a question we received: "If I accelerate gently to 65 mph, it saves gas, right?"

Well, yes, it does. But that's missing the forest for one single, somewhat scrawny tree.

The important thing to know is that above about 30 or 40 mph, it's all about aerodynamic drag. That's when you start to use more energy just to push aside air than you do to move the mass of your car.

The energy (e.g. gallons of gasoline) required to move your car 1 mile is roughly proportional to the square of the speed at which you move it.

For instance, it takes about twice as much energy to drive a mile at 65 mph as if you drove the same mile 20 mph slower, at 45 mph. That means you burn essentially twice as much gasoline to cover that mile faster.

BMW's Aerodynamic Test Center near Munich

BMW's Aerodynamic Test Center near Munich

And you double it again if you go up from 65 mph to speeds illegal pretty much anywhere in the States, like 90 mph.

There are several caveats (which we won't go into) that affect the actual consumption, but those are the physics that underlie your gas consumption.

And it's why, in general, automakers are paying more attention than ever to making their cars aerodynamically smoother.

Hybrids like the Toyota Prius, and plug-in vehicles like the Chevy Volt and Nissan Leaf, in particular, usually have lower drag coefficients than more conventional cars.

Traffic engineers will tell you that during the last gas-price spike, in the summer of 2008, average speeds on Interstate highways slowed down a bit. Unconsciously, perhaps, drivers were trying to save fuel.

So, if you're really interested in stretching your gas dollar and not wasting a drop of precious fuel ... not only should you "drive like there's an egg between your foot and the accelerator pedal," you should also obey the speed limit.


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