Slow Down and Save Up
December 9, 2009 at 2:06 pm
There are plenty of things you can do to save money on gas, one thing stands out – drive slower.
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 makes sense. 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.
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.
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.
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.
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.
Of course driving 10 mph hour faster can save you time on a long trip, so whether or not the higher cost of gas matters more than your time is up to you.
[Source AOL News]
