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.