High gas prices have made drivers more aware than ever of the cost of a fill-up and the fuel economy of their current cars, but those who are using the fuel economy calculator built into their cars to track their miles per gallon are in for a surprise. The gauge, which is usually part of your car's trip computer, is probably wrong.

A recent study by independent car information site Edmunds.com found that the gauges overestimated fuel economy by 5.5% on average, with one gauge overestimating by 19%.

While a 5.5% overestimation may not seem significant, it can really add up over time. For example, take a car that shows 25 mpg on its fuel economy gauge, but it's really off by 5.5%. That would total 132 gallons of "unreported fuel use" over five years of driving, assuming you drive the 15,000 miles per year that is average in the U.S., Edmunds.com says. That's at least a $500 additional cost in gas and possibly more, depending on the cost of gasoline.

The discrepancy grows in cars that have worse average fuel economy, even when the percentage of overestimation stays the same. For example, Edmunds.com says an SUV that gets 12.5 mpg on the fuel economy gauge and is overestimating by 5.5% would result in more than $1,000 in additional fuel costs over five years of average driving.

And, if you drive a car that is overestimating its fuel economy by even more, your cost could be much higher.

One reason the on-board gauges are inaccurate is that ethanol levels in gasoline can vary widely, and ethanol reduces the energy the gasoline has to propel the car. The on-board gauges are programmed to assume certain ethanol percentages to make the calculations, but those numbers may not be accurate, depending on where you fill up.

Furthermore, it's entirely up to each automaker what ethanol percentage is used or even if one is used at all. That could make one gauge off by a little and another off by a lot -- with the same driver who drives the same way and fills up at the same station.

The best way to get an accurate picture of your car's fuel economy and to budget your gasoline costs is to do some old-fashioned math. Log the miles you drove between fill-ups, and divide that by the number of gallons of your fill-up. By keeping a log for a while, you may notice trends associated with your driving and with the octane rating you use or where you fill up.

A pen and paper will do the trick. However, there are a variety of websites and mobile apps that will do the math for you and keep track of the data. Do a search for "gas mileage tracker" on the Internet or your cellphone's app store to find an electronic version that suits you.

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