Tips to Save Gas

Research shows that slowing down can save you a lot of fuel

By PETER VALDES-DAPENA

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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.

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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.)

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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.

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417 comments

Mrarmer 03:14:03 AM Jul 26 2008

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Oops this was in responce to Evilcamero. if your are doing 65 in a 35 and a pedestrian steps in your path, there is no way you could stop in time. thus possibly killing someone. Hope you can see the logic in this. Speed does Kill people. Especially when people disobey the law. Check out www.safespeed.org.uk/thatad.html

Mrarmer 03:11:28 AM Jul 26 2008

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if your are doing 65 in a 35 and a pedestrian steps in your path, there is no way you could stop in time. thus possibly killing someone. Hope you can see the logic in this. Speed does Kill people. Especially when people disobey the law. Check out www.safespeed.org.uk/thatad.html

Deserthspd 10:33:04 PM Jul 08 2008

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It is true, drive slower mean saving gas...If you think it is possible, try it one time when you are on trip or mini vacation and you will see the different. I used to haul ass from my house to work at 125 miles away and did not realize that I used more gas until one day, I heard on the radio about stay at 65 would save gas. The next day, I tried it sure enough, I get 32 miles per gallon from V^ Honda Accord, go figure...Try it....

Japeruzzi 10:49:15 AM Jun 28 2008

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Evilcamero is an idiot.

Pwwachtler 11:13:04 PM Jun 11 2008

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I totally agree with the Camaro guy; I have one and 75 is indeed an ideal speed for this car. Any slower and i have to drop a gear so as not to lug the engine!

JeanM617 11:26:46 AM May 26 2008

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I agree with slowing down to save fuel - I now own a more fuel efficient vehicle, not a hybrid mind you, and have driven 100 miles on it since I gassed up to full a couple of days ago - it has only dropped a little and isn't even at the 3/4 mark yet...which means if I keep driving @50 to 55 I can go at least 200 hundred more miles - on one tank of gas. That is why it was worth it for me to trade in my less fuel efficient vehicle a couple of weeks ago. Whenever someone passes me going 70 MPH I don't think "Wow, look at them, they are soooo cool." - I think they will have to gas up that much sooner than I and how much $ they are wasting at the pump.

EvilCamaroSS 12:07:48 PM May 25 2008

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You people are all out of ur frickin minds, "oh no! don't speed it kills people!" WRONG Ignorance kills people. Pushing the limit kills people, Distractions kill people. not speed. Speed does not kill people. I can safely navigate my car @65mph in a 35MPH zone. As i have done so every day for the last 2 years. Just because i drive fast does not mean i don't drive safe, and if you idiots would realize that all cars have a prime fuel efficiency speed you would be anxious to get to that speed faster to save gas. Most cars have peak fuel efficiency at 50-60mph so while your sitting there slugging the speed limit in a 30mph zone your costing yourself more money! Then you have cars like my 98 Camaro SS w/ well over 400 wheel horse. which reaches a peak fuel efficiency at 75 mph. @ 75mph in 6th gear I'm at 1,750 Rpms. So if you wanna continue to waste your gas at the posted speed limit get the hell out of my cay, cause every time you hit your breaks to slow down for that turn that can be easi

RShiro 01:23:31 AM May 24 2008

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In the phoenix, scottsdale area I have been making a short study on speeders and gas saving. Driving for three months at exact speed limits I watched as 100% of the drivers past me at 5 to 10 miles faster. I also was able to anticipate and drift down to traffic lights. 100% of the drivers past me and jammed on their brakes at the red lights. I am convinced that no one really cares about gas prices and saving costs by driving at speed limits. Also huge freight trucks past me every time. In virginia I drove 90 miles at the speed limit. All auto and huge freight trucks past me and were driving @ 75 miles an hour in a 65 mile zone. To keep taxes down states and local governments should double speeding tickets. they could take in one billion extra dollars per year. speeders should be charged $500.00 per ticket. Oil by 2012 will be over 200 dollars a barrel and gas will be increased to $8 to $10 per gallon. Who Cares? Speeders don't.

Pashon8man 09:47:52 PM May 22 2008

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Slowing down is not only gas or money waste but it can also waste a precious life or lives. When one life ends in a traffic accident it starts unlimited crises to the whole family. Let's not forget that we also buy oil from non Arab countires so suck it up. This is not a religious talk about Arabs or Muslims it is all about savings and saving everything from money to lives.