by Peter Valdes-Dapena
Speeding on the highway adds a surprising amount to your fuel costs.
With gas prices rising, gas-saving advice abounds: Drive more gently, don't carry extra stuff in your trunk, combine your shopping trips.
This is all sound advice but there's one driving tip that will probably save you more gas than all the others, especially if you spend a lot of time on the highway: Slow down. In a typical family sedan, every 10 miles per hour you drive over 60 is like the price of gasoline going up about 54 cents a gallon. That figure will be even higher for less fuel-efficient vehicles that go fewer miles on a gallon to start with.
The reason is as clear as the air around you.
When cruising on the highway, your car will be in its highest gear with the engine humming along at relatively low rpm's. All your car needs to do is maintain its speed by overcoming the combined friction of its own
moving parts, the tires on the road surface and, most of all, the air flowing around, over and under it.
Pushing air around actually takes up about 40% of a car's energy at highway speeds, according toRoger Clark, a fuel economy engineer for General Motors.
Traveling faster makes the job even harder. More air builds up in
front of the vehicle, and the low pressure "hole" trailing behind gets
bigger, too. Together, these create an increasing suction that tends to
pull back harder and harder the faster you drive. The increase is actually exponential, meaning wind resistance rises much more steeply between 70 and 80 mph than it does between 50 and 60.
Every 10 mph faster reduces fuel economy by about 4 mpg, a figure that remains fairly constant regardless of vehicle size, Clark said. (It might seem that a larger vehicle, with more aerodynamic drag, would see more of an impact. But larger vehicles also tend to have larger, more powerful engines that can more easily cope with the added load.)
That's where that 54 cents a gallon estimate comes from. If a car
gets 28 mpg at 65 mph, driving it at 75 would drop that to 24 mpg. Fuel costs over 100 miles, for example - estimated at $3.25 a gallon - would increase by $1.93, or the cost of an additional 0.6 gallons of gas. That would be like paying 54 cents a gallon more for each of the 3.6 gallons used at 65 mph. That per-gallon price difference remains constant over any distance.
Engineers at Consumer Reports magazine tested this theory by driving a Toyota Camry sedan and a Mercury Mountaineer SUV at various set cruising speeds on a stretch of flat highway. Driving the Camry at 75 mph instead of 65 dropped fuel economy from 35 mpg to 30. For the Mountaineer, fuel economy dropped from 21 to 18.
