(May 16, 2011) SANTA MONICA, Calif. — Fuel economy gauges, which now come standard in 92 percent of new cars, may be over-representing your car's fuel efficiency, according to tests conducted by editors at Edmunds.com.

A total of 14 tests in seven vehicles found that the MPG gauges were 5.5 percent inaccurate, on average.

In the case of the 2010 Ford Escape Hybrid, the MPG claims were overstated by as much as 19 percent and for the 2010 Volkswagen Jetta SportWagen TDI, the gauge was off by 16 percent.

"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 it adds up over the typical five-year period of car ownership," said Phil Reed, senior consumer advice editor at Edmunds.com.

"If a driver uses the fuel economy meter as the basis for budgeting a 25 mpg car, 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."

Many car companies contacted by Edmunds.com did not respond to questions about Edmunds' findings. A GM spokesman, however, suggested that varying levels of ethanol mixed in gasoline may have an adverse impact on a car's fuel economy, since the substance provides less energy.

A Honda spokesman said that drivers should use fuel economy gauges as "a driving-efficiency tool, not a precise measurement of fuel economy."

To get a true estimate of your car's fuel economy, Edmunds.com suggests taking the matter — quite literally — in your own hands. There are several iPhone apps that can help you log and monitor your car's fuel economy.

And then there's the old fashioned way — use a pencil and notebook and a little bit of fourth-grade math. Seems that's the way fuel economy was figured during the first 100 years of the automobile.

For more details on Edmunds.com's MPG gauge findings, visit "Your Fuel Gauge is Fibbing."