Slow down a little, save a lot of gas

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NEW YORK (CNNMoney.com) -- 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 to Roger Clark, a fuel economy engineer for General Motors (GM, Fortune 500).

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.

Over the course of a 400-mile road trip, the Camry driver would spend about $6.19 more on gas at the higher speed and Mountaineer driver would spend an extra $10.32.

Driving even slower, say 55 mph, could save slightly more gas. In fact, the old national 55 mph speed limit, instituted in 1974, was a response to the period's energy crisis.

It was about more than just high gas prices, though. The crisis of the time involved literal gasoline shortages due to an international embargo. Gas stations were sometimes left with none to sell, and gas sales had to be rationed. The crisis passed, but the national 55 mph speed limit stayed on the books until the law was loosened in the 1980s. It was finally dropped altogether in 1995. (The law stuck around more because of an apparent safety benefit than for fuel saving.)

Despite today's high gas prices, don't expect to see a return to the national 55 mph speed limit. The law was unpopular in its day, and higher speeds have become so institutionalized that even the Environmental Protection Agency's fuel economy test cycle now includes speeds of up to 80 mph.

Driving 10 miles per hour faster, assuming you don't lose time getting pulled over for a speeding ticket, does have the advantage of getting you to your destination 50 minutes sooner on that 400 mile trip. Whether that time difference is worth the added cost and risk is, ultimately, up to you.