High-performance vehicles get relatively more efficient at high speeds, while hybrid efficiency remains constant. Aggressive driving burns more gas for both.

On Jan.12, the Environmental Protection Agency announced that it would update its fuel economy calculations, starting with vehicles hitting showrooms in 2007. The new EPA formula will take aggressive driving and other factors into greater consideration. It’s unknown how the change will affect window sticker mpg numbers for hybrid cars. A report issued last year by the Department of Energy may provide some clues.

In April 2005, researchers from the DOE’s Argonne National Laboratory presented a paper entitled, "Investigating Vehicle Fuel Economy Robustness of Conventional and Hybrid Electric Vehicles." The study showed that hybrid fuel economy suffers under more demanding driving conditions, including high speeds and/or fast acceleration. On the other hand, more aggressive driving patterns could produce higher rather than lower efficiencies in some conventional vehicles.

The ATDS and US06 driving test cycles, used by the EPA for evaluating emissions, are much more aggressive than the EPA’s current cycles used to produce mpg figures. All vehicles, except for the Jaguar XJ8, suffered under more demanding driving conditions.

Hybrids—Already Efficient—Have Little Room to Improve

Michael Duoba, the study’s principal investigator, makes a distinction between efficiency—the amount of work you get with respect to how much energy is invested—and the overall fuel required for a specific trip. He said, "In more aggressive driving, virtually any car will actually achieve higher efficiencies in terms of thermodynamics, but driving faster requires more power and energy, thus more fuel is required."

Duoba explained that hybrids get very high efficiencies and perform very well under light loads. He added, "As soon as you drive faster, the hybrids simply don’t have anywhere to go. They are already very efficient. "

The most dramatic comparison in the multi-vehicle DOE study was analyzing the differences between the Toyota Prius—the quintessential hybrid—and the Jaguar XJ8, a high-performance vehicle. To establish a baseline for efficiency, the DOE researchers searched for the speed at which each vehicle was the most efficient. They used a treadmill-like device called dynamometer to run the vehicles at a constant speed for 30 continuous minutes. The most efficient speed for the Toyota Prius was 20 mph, and its least efficient level was 70 mph.

Results for the Jaguar were the opposite. At a constant speed of 20 mph, when the Prius obtained a fuel economy of over 140 mpg, the Jaguar was recorded at a level of 37 mpg. Every additional 10 mph helped the Jaguar’s efficiency until it reached its peak of 40 mpg at 50 mph. Duoba said, "Because the Jaguar is so inefficient at low speeds, you have to drive faster to improve its fuel economy."

Hybrid Efficiency Remains Constant

To test patterns more consistent with actual road conditions, the researchers used the EPA’s well-known and much criticized urban driving cycle. To simulate various levels of aggressive driving, they intensified that cycle in a series of four steps until they reached an average speed of 40.0 mph, with a maximum speed of 79.4 mph. (Don’t try this at home.) By contrast, the average speed for the EPA’s current urban cycle is 24.1 mph.

When the cycle was pushed to 1.4 times the load of the current EPA test, the Toyota Prius was the only vehicle to maintain high efficiencies at nearly the same level as the standard urban cycle. All of the conventional vehicles became slightly more efficient in more demanding situations. "The Jaguar’s engine, which is way oversized, is usually being driven at lower loads, which is very inefficient," explained Duoba. "When it comes to hybrids, the efficiency remains constant."

The Toyota Prius, When pushed to 1.4 times the load of the current EPA test, the Toyota Prius was the only vehicle to maintain high efficiencies at nearly the same level as the standard urban cycle.

Hybrid cars utilize complicated integrated system that allows manufacturers to optimize the vehicle’s efficiency for a specific drive cycle. Duoba thinks the DOE test results show how Toyota made a deliberate decision to size the Prius’s hybrid components for the low-speed, stop-and-go driving of the EPA urban driving cycle. (Could Toyota modify the design for the new EPA mileage formula?) Simpler and larger engine systems, indicative of past generations of automotive technology, don’t have the advanced technology or controls to adapt to different driving patterns.

The DOE study and changes in EPA policy are unlikely to satisfy car shoppers quest for definitive information about hybrid fuel economy. "We fool ourselves to say that any car has a specific miles per gallon. But the general rule is the more aggressive you drive a hybrid, the more your high fuel economy will suffer when compared to conventional cars. It’s all relative."