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January 10, 2005

Hybrid Cars and Misleading Gas Mileage Tests

Truth about Cars, via Clayton Cramer has some interesting things to say about hybrid cars:
Buyers pay a large premium for a hybrid Escape or a Prius, presuming that the increased fuel mileage makes them a better environmental citizen. While there’s no question that the Toyota, Honda and Ford hybrids are more fuel efficient than their conventionally powered equivalents, the difference is nowhere near as great as the Environmental Protection Agency (EPA) numbers suggest.

Because of the low speeds involved, the city portion of the EPA’s test is accomplished in battery-only mode. As the gasoline engine is off- line for a significant part of the test, the eventual mileage figure is grossly inflated. The test fails to consider the fuel needed to recharge the batteries later on. What’s more, all energy-draining, electrically- powered accessories (including AC) are switched off during both the urban and highway tests. These variables contribute to the huge discrepancy between the EPA’s official numbers and hybrid owners’ real world experience.

Few people realize that a hybrid’s power train adds roughly 10% to the weight of a car. Even fewer realize that manufacturers try to offset the weight penalty-- and add to the hybrid’s headline-grabbing mileage figures-- by the extensive use of non-hybrid gas-saving technology. Engine shut-off at idle, electric power steering, harder and reduced rolling resistance tires (at the expense of comfort and traction), reduced option content, reduced engine performance, and, in the case of the Ford, a continuously variable transmission (CVT) all help raise the cars’ overall efficiency.

Thus, there are three main points: 1. During much of the EPA test, only the batteries are used, but they need to recharged later by burning gasoline. 2. Electrical accessories (including lights, not mentioned in the article) in the hybrid are powered less efficiently by the hybrid battery than by normal batteries (The article doesn't say this outright, and it might be wrong, but if it isn't, the fact that accessories are shut off is no more misleading for the hybrid than for regular cars.) 3. Much of the hybrid's gas mileage is legitimate but achieved by means older than the hybrid engine.

Posted by erasmuse at January 10, 2005 11:13 AM

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