Understanding Fuel Economy Ratings

Fuel economy ratings rarely came up in the days when fuel was cheap. With the rise in gas prices came a corresponding rise in awareness fuel economy awareness. To determine a vehicle's fuel economy rating, both manufacturers and the Environmental Protection Agency (EPA) run laboratory tests using preset conditions. The manufacturer will test all of its vehicles, while the EPA chooses 15 to 20 percent of those vehicles to confirm the manufacturer's results.

To test a vehicle's fuel economy, a professional driver leads the vehicle over a preset pattern that is the same for all vehicles and takes place on a dynamometer (think huge treadmill). The testers recreate conditions that vehicles will encounter on the road, such as different wind speeds, temperatures and different driving speeds.

The city test involves the vehicle averaging 20mph while making frequent stops creating a lot of accelerating, when vehicles use the most fuel. The highway test simulates standard high conditions with no stopping and very little braking. The driver averages 48mph. When conducting the high speed test, the driver will brake hard and accelerate hard, topping out at 80mph.

The temperature is the variable being tested in both the AC (air conditioning) and cold temperature tests. The heat is cranked up to 95 degrees Fahrenheit and the AC is turned on for one and the thermostat is dropped to 20 degrees Fahrenheit for the other. They take the results of these tests to calculate the vehicle's estimated fuel economy.

This calculation gives you the vehicle's estimated highway and city mpg (miles per gallon). As of 2008, a vehicle's sticker will also give a potential buyer the estimated annual fuel cost based on the average cost of fuel. It should be noted that the actual annual fuel cost depends on the owners driving habits, whether the climate is warm or cold, whether the vehicle travels mostly in city conditions or highway conditions and what the actual cost of fuel is per gallon.