Gas Mileage: Why MPG Numbers Really Matter
(Page 2 of 5)
August/September 2007
By James Kliesch
With that in mind, the Agency’s most recent modification incorporates the results of three additional tests: cold conditions, use of air conditioning and high speed/aggressive driving. These tests, already conducted on vehicles to measure tailpipe exhaust, are now combined with the original city and highway driving tests to make a new “five-cycle” test that provides a more accurate assessment of fuel economy.
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So what does the change mean to you and me? For city mpg, the EPA says the numbers of most conventional vehicles, including diesels, will drop between 8 percent and 15 percent. The city mpg for hybrids will decrease between 20 percent and 30 percent. The highway mpg of most vehicles will drop from 5 percent to 15 percent; the numbers for certain hybrids may decline between 10 percent and 20 percent. (For examples, see the chart below.)
Why are hybrids more susceptible to inflated mpg estimates? According to John German, manager of environmental and energy analysis with American Honda Motor Co., it has less to do with hybrid technology than with high-mpg designs. For starters, high-mpg vehicles have lower baseline fuel consumption. Thus, unlike muscle cars with oversized engines that offer additional horsepower on tap, high-mpg vehicles are more susceptible to mpg declines when they encounter higher loads, such as aggressive driving or the use of air conditioning. So by including these additional loads in the new math, German says, the results for hybrids and other efficient vehicles will be more heavily affected, even though their “increase” in fuel consumed with the new mpg estimates is no larger than the “increase” for other, less-efficient vehicles. And, Millett says, “you’re still going to get better mileage in a hybrid.”
Of course, the actual fuel economies of the vehicles aren’t changing at all. The new estimates that appear on window labels simply offer more accurate “truth in advertising.” This will help consumers in comparison shopping, and give them a more accurate idea of what their annual fuel costs would be with a specific vehicle.
The new numbers also won’t change fuel consumption, global warming emissions or even automakers’ federal mpg requirements. The math for CAFE regulations was set into law in the ’70s; the new math for fuel economy affects only what you see on window labels. Because of this, the new estimates will have no bearing on the auto industry’s mpg obligations.
Also bear in mind that the fine print “your mileage may vary” remains true, even with the new math. It is a near-impossible task for the EPA to simulate the driving styles and driving conditions of everybody, everywhere — Death Valley in the middle of summer is a far cry from Boston in the dead of winter. Where and how a vehicle is driven; condition and care of the vehicle; the type of tires; climate and weather conditions; and use of accessories such as air-conditioning are but a handful of factors that will uniquely alter any given vehicle’s real-world mpg.
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