Experts: Don't Rely On Fuel Efficiency Gauges

Almost any car manufactured in the last couple of years has a gauge that measure's the vehicle's fuel economy, showing you how many miles to the gallon you are getting.

But if you put all your faith in these gauges, you might not be getting nearly the fuel efficiency you think you are.

The editors at Edmunds.com conducted a total of 14 tests in seven vehicles and found that the Miles Per Gallon (MPG) gauges were 5.5 percent inaccurate, on average.

Double-digit discrepancies

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."

The Edmunds experts said they did not get much help from most car companies when they inquired about the discrepancy. However, they said General Motors did respond,

suggesting 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.

Not a precise measurement

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

The Edmunds experts say there are several iPhone apps that can help you log and monitor your car's fuel economy.

And, of course, drivers can measure mileage the old fashioned, tried-and-true way. Simply log the miles between each fill-up, dividing the miles by the number of gallons it takes to fill the tank. You can improve your gas mileage by keeping your tires inflated with the proper pressure and your engine tuned up.