The Manufacturers' Responses

Edmunds contacted all the manufacturers with vehicles represented in the two tests and asked for an explanation for these consistently overly optimistic fuel economy readings. A Ford spokesman replied that it was too difficult to find the right engineer to respond to this question. BMW and Volkswagen did not respond to requests from Edmunds.

Roger Clark, senior manager of GM's energy center, explains that the fuel economy gauge makes a calculation by counting the number and duration of pulses made by the fuel injectors as they squirt gasoline into the combustion chambers of the engine. The onboard computer system divides the distance the car travels by this estimated fuel consumption.

Clark says the gauge is "dead nuts accurate" — if you consider all the variables at work during driving, including temperature, driving conditions and driving style. The biggest fluctuation occurs because ethanol, which is blended with gasoline in varying amounts, contains less energy.

"When you fill up, you are paying for a gallon of gas, but the energy in that gas varies significantly," Clark says. This means that while the car's computer assumes the gasoline is providing energy to drive a certain distance, the fuel might have less energy and not propel the car as far.

The 5.5 percent average variation in the vehicles Edmunds tested "seems like a perfectly reasonable range to me," says Paul Williamsen, national manager of the Lexus College, where his responsibilities include service training for Lexus staff, dealers and corporate personnel. "I can't imagine any reason that any automaker would want to make drivers think they can get better fuel economy than they were getting," Williamsen adds.

Honda spokesman Chris Martin wouldn't comment on the accuracy of Honda gauges, saying he needed to do more research to give a good answer. However, he defends fuel economy gauges in general, and says that people should use them as "a driving-efficiency tool, not a precise measurement of fuel economy."

The gauges make their point best when they utilize symbols rather than numbers, Martin says. Many new Honda models have gauges that display color changes to reflect how efficiently someone is driving. This method is more effective at helping drivers learn to drive efficiently than is other feedback that might require more attention to understand, he says.

Mini's gauges are "very accurate" because they use real-time information, says Mini Product Manager Vinnie Kung. However, he adds that inaccuracies can come from such factors as fuels and even fuel tanks, which expand during warm weather. For instance, Kung notes that in the summer a Mini might be able to hold 14 gallons of fuel in a tank whose labeled capacity is 13.2 gallons. In the winter, it might only hold 12.9 gallons. This variation affects a variety of readings, including mpg and the "distance to empty" reading that shows how much range is left.