How is vehicle fuel economy (mpg) measured in the US ?

The US EPA requires vehicle fuel consumption to be measured using lab chassis dynamometer testing. A representative vehicle is run over five test cycles, listed below along with the respective ambient temperatures:

1. FTP-75 at 75 F: Represents city driving. Includes 3 measurements called bags (because the emissions are collected in bags !), with the first bag after a cold start (>12 hour overnight “soak”) and a hot start with engine off for only 10 min.
2. HFET at 75 F: Highway fuel economy test
3. US06 at 75 F: High speed and aggressive urban driving
4. SC03 at 95 F: Represents the impact of air conditioner use at high temperatures
5. Cold FTP at 20 F : Impact of cold ambient temperatures

The above tests measure the fuel consumption over a wide range of driving conditions and provide a realistic assessment of on-road performance to the consumer. Note that till a few years back, only two cycles were used to measure the fuel economy: FTP-75 and HFET.

Miles per gallon numbers from the above 5 cycles are combined to give composite fuel economy figures for city and highway driving each. The formula is rather messy, and can be found on page 92 of the PDF link below.

Here’s a handy tool, courtesy of the U.S. EPA, to find and compare the MPG of vehicles, a handy tool when purchasing your next car.

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Also see : Global passenger car fuel economy standards

Further reading

• EPA Fuel Economy Testing Database
• EPA document on revised fuel economy calculations (PDF)