Measuring fuel economy

The EPA measures the fuel economy of most cars on the road in the US at fueleconomy.gov These estimates are computed using a fairly detailed procedure:

• Drive the vehicle on a dynamometer, whose resistance is adjusted to account for air resistance and vehicle weight.
• For city estimates: drive the vehicle for about 30 minutes at an average speed of 21 mph, covering a distance of 11 miles.
• For highway estimates: drive the vehicle for about 10 minutes, at an average speed of 48 mpg, covering a distance of 10 miles.
• Additional specialized tests are done for high-speed driving, air conditioning, and so on.

For the city estimate, note that the 21 mph average speed can be considerably higher than the speed you might experience in heavy stop-and-go traffic. If you live in an area where there are persistent traffic delays, you can easily fall far short of this 21 mph average, so your gas mileage will likely be worse than the EPA's estimate.

Let's look at a concrete example: the 2013 VW Golf, 2.5 L gasoline engine:

The EPA estimates that this car will achieve 24 MPG in the city, 31 MPG on the highway, and a "blended" rate of 26 MPG. This blended rate depends on your personal mix of city/highway driving: the EPA default is 45% highway, 55% city, but you can personalize it to reflect your circumstances.

Note that, although the default city/highway mix is pretty close to 50%, the blended MPG is a lot closer to the city estimate than the highway estimate. I discuss this in a separate blog post, but the basic idea is that, for any car, the city MPG matters a lot more than the highway MPG towards the final blended MPG.

Are these EPA estimates "accurate", that is will they match what you, as the driver of the car, will actually see in terms of how much fuel you are using? The EPA website allows users to report their own MPG numbers, using the "Your MPG" feature. For the car above, you can see that 1 driver of this particular car reports an actual MPG of 27, which is not far from the official EPA estimate.

Using data from only 1 driver is not very accurate, however, so there are websites which aggregate far more data, from many more drivers, to provide more accurate fuel consumption estimates:

• fuelly (World-wide, although seems to be heavily skewed towards the US)
• honestjohn (UK): uses imperial gallon in its MPG definition, which is different from a US gallon
• spritmonitor (Germany): uses liters/100km for fuel efficiency

In the case of the above vehicle, fuelly reports a comparable average of 26.2 MPG, from 11 different cars:

When evaluating a new car, be sure to look at both the EPA estimates as well as user-reported estimates, to get a more accurate picture of the vehicle's fuel efficiency. Better yet, report your own fuel efficiency estimates at the appropriate site above, so that everyone else can benefit from your information.