Your Fuel Economy Gauge Is Fibbing

As gas prices rise, drivers are paying closer attention to the fuel economy gauges that are found in most late-model cars as part of the trip computer. The only problem is that the gauges are inaccurate. In fact, Edmunds testing reveals that one such gauge claimed fuel economy 19 percent higher than the actual result.

Across two tests in seven different vehicles, the gauges were 5.5 percent inaccurate on average, according to data gathered by the editors at Edmunds.com.

The editors noted such optimistic estimates from fuel economy gauges during our 2009 and 2010 "Fuel-Sipper Smackdown." In two separate tests, editors drove five fuel-efficient cars from Los Angeles to Las Vegas and back under three different driving conditions: back roads (45-60 mph), city streets (stop and go) and highway (70-75 mph). During the tests (a total distance of more than 1,550 miles was accumulated by each car), the editors measured fuel economy by calculating how much gas was required to go a certain distance and comparing that to the reading on the fuel economy gauge.

Gauges Come Standard — and Skew High

Fuel economy gauges, which show both average and current fuel economy, are standard equipment in 92 percent of 2011 vehicles, according to Edmunds data. By resetting the gauge when refueling, a driver sees what kind of fuel economy the vehicle delivers. Drivers can change their driving style and see if this improves fuel economy.

The individual inaccuracies in Edmunds testing were as high as 19 percent for the 2010 Ford Escape Hybrid and 16 percent for the diesel-powered 2010 Volkswagen Jetta SportWagen TDI. This means that the Escape was getting 5 mpg less than the gauge indicated, while the Jetta was getting 5.7 mpg less. The test also included a 2009 Mini Cooper, 2010 Ford Fusion Hybrid, 2010 Honda Insight, 2010 Toyota Prius, 2010 Toyota Highlander Hybrid, 2010 GMC Terrain and 2010 BMW X5 xDrive 35d.

A 5.5 percent error in a car's estimated fuel usage might not seem like a big deal over a single tank of gas, but over the typical five-year period of car ownership, it adds up. Take a car that shows 25 mpg on its fuel economy gauge, but which actually is consuming 5.5 percent more. Extend that over five years of driving at 15,000 miles per year and you get 132 gallons of unreported fuel use. That represents a substantial amount of money.

If a driver uses the fuel economy meter as the basis for budgeting, he would plan for five-year fuel costs of $12,000 (assuming fuel stays at $4 per gallon). In reality, the figure would be $12,660. The discrepancy is even larger for vehicles that have worse average fuel economy. The driver of an SUV or pickup that averages 12.5 mpg according to its fuel economy gauge might budget $24,000 for fuel. The actual cost would be $25,320 — a difference of $1,320.

But since our testing discovered that some fuel economy gauges are far more optimistic than the 5.5 percent average, some scenarios are even worse. It's entirely possible that our hypothetical 25 mpg car could consume 500 more gallons than predicted by the gauge over the typical five-year ownership period. That's $2,000, assuming $4-per-gallon gas. A thirstier truck or SUV might consume 1,000 additional gallons, adding up to $4,000 over five years. And, of course, it would be even more if the price of gas rises during that period.

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.

Don't Rely on the Readings

Steve Mazor, chief auto engineer for the Auto Club of Southern California, has a different explanation for the consistently high readings for the fuel economy gauges. The gauge "assumes it is a perfectly operating vehicle — and it isn't." For example, he says fuel injectors can become clogged and not deliver as much fuel as the gauge assumes.

Mazor certainly doesn't see any conspiracy by automakers to mislead the public with consistently high numbers, but says of the gauges: "We tell people not to trust them except as a comparative tool." In other words, a driver could use the gauge to see differences produced by changing his or her driving style.

Dan Edmunds, director of vehicle testing for Edmunds.com, thinks there's a better way to express the variations in fuel economy. He believes that rather than consistently present a best-case mpg figure to drivers, the calculations that power the gauge readout should incorporate some of the variables noted by the experts above. That's better than presenting drivers with "figures that are never under and always over" the actual fuel economy, he says.

And despite protestations to the contrary, Dan Edmunds says there is an incentive for carmakers to present overly optimistic mpg feedback to their customers. "Because window sticker ratings and mpg advertising claims are hard to match in real life, fuel economy is one of those things that is often ranked 'below expectations' on owner feedback surveys like J.D. Power's Initial Quality Survey," he says.

Whatever the reason for their inaccuracies, it seems that fuel economy gauges should have this label: "Your actual mileage may vary." And Dan Edmunds recommends using a second source for recording fuel economy, such as joining Fuelly.com and logging every tank of gas to get a more accurate reading. But for the technologically challenged, he recommends a hands-on approach. "Grab a pen and paper, keep track of the data yourself and come up with your own numbers."