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 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.
(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.
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