What does “EPA Estimated Fuel Economy” mean?

These are questions that comes up again and again in the car business:

  • What does the EPA Estimated Fuel Economy really mean?
  • Is that the actual, real-world fuel economy that I should expect to get in my vehicle?
  • If it isn’t, why does it matter?

Hopefully, this blog post will help to clear up some questions for you about what the EPA estimates really mean, how they are calculated, how they are useful, and in what ways they aren’t so useful.

How are EPA Fuel Economy Estimates calculated?

The first thing to understand is how the EPA fuel economy estimates are truly calculated. There are two numbers that are fairly well known: one is for fuel economy for city/urban driving, and one is for highway driving. City driving is generally understood as stop-and-go patterns, with short bursts of activity, getting up to around 30 or so miles per hour, just like you would if you were driving somewhere with medium traffic and stop signs or traffic lights every couple hundred yards or so. Highway driving is generally understood to be a longer distance – at least five or ten miles – with sustained speeds of above 60 miles per hour.

This is basically what the EPA tests, but it can’t do it in a real world setting (like actually driving in a city or on the highway), because there are too many uncontrolled variables, like traffic, weather, timing of traffic lights, and the exact speed and acceleration of the driver.

So, back in the 1970’s, the EPA constructed a series of “Driving Cycles” which the driver must follow for the test. These cycles are designed to simulate city and highway driving conditions, as well as accounting for things like the use of air conditioning. The vehicles are placed on a dynamometer like the one shown below, so that the test can be monitored for accuracy, as well as giving the driver more exact control over acceleration and stopping.

http://www.fueleconomy.gov/feg/how_tested.shtml

Car on Dynamometer – Courtesy of fueleconomy.gov

By using a dynamometer, a lot of the guesswork is taken out of the testing process, and every car is subject to exactly the same conditions.

What do the tests actually look like?

Below are the five driving cycle tests that are done to calculate EPA fuel economy estimates:

EPA Fuel Economy Driving Cycle - City

EPA Fuel Economy Driving Cycle – City

This test is done to measure city driving. This test involves frequent stop-and-go driving, with peaks generally around 30 miles per hour. The car’s engine starts cold, to simulate real-world conditions, and temperatures in the lab are kept between 68°F and 86°F.

EPA Fuel Economy Driving Cycle - Highway

EPA Fuel Economy Driving Cycle – Highway – Courtesy of fueleconomy.gov

This test simulates highway driving. The vehicle’s engine starts warm and averages 48 miles per hour, peaking at around 60 miles per hour.

EPA Fuel Economy Driving Cycle - High Speed

EPA Fuel Economy Driving Cycle – High Speed – Courtesy of fueleconomy.gov

This is one of three tests added in 2008 to more accurately reflect real-world driving scenarios. This test simulates sustained high-speed driving, including a long sustained period of driving above 55 mph, and peaking at 80 mph.

EPA Fuel Economy Driving Cycle - Air Conditioning

EPA Fuel Economy Driving Cycle – Air Conditioning – Courtesy of fueleconomy.gov

This test is the second test added in 2008, and reflects the use of air conditioning. The driving cycle is half-way in between city and highway driving, with some stop-and-go driving, as well as some sustained driving, with a peak of about 55 MPH. For this test, the air conditioning is running continuously, the engine is already warm, and the temperature of the laboratory is kept at 95°F.

EPA Fuel Economy Driving Cycle - Cold Temperature

EPA Fuel Economy Driving Cycle – Cold Temperature – Courtesy of fueleconomy.gov

The final test, added in 2008, is the same driving cycle as the city driving test, but this time, the laboratory temperature is kept at 20°F. Because engines are less efficient when they are not up to temperature, this is a more accurate reflection of fuel economy in colder climates and during the winter.

Why is my real-world fuel economy different from the EPA Estimates?

There are a number of reasons for discrepancies between the EPA estimates for fuel economy and real-world fuel economy. Many of them have to do with driving habits, vehicle maintenance, and other factors that are in the control of the driver. You can check out our monthly “Fuel Saver Tips” blog posts for ideas on increasing your own fuel economy.

A large factor is the actual driving cycles themselves. These tests were designed in the 1970’s, back when the maximum highway speed nationwide was 55 miles per hour, and the tests are reflective of that. Since then, a test has been added which reaches a maximum speed of around 80 Miles per hour, but this is only one test of five that is factored into the calculations.

In addition, there are a number of other factors that are strictly controlled in the tests. For one, the tests use 100% pure gasoline. Any time ethanol is added to gasoline, it can lower the fuel economy of a vehicle. If you are driving flex-fuel vehicle, and using E85 fuel, which is 85% ethanol and only 15% gasoline, you will notice a significant drop in fuel economy compared to using even a 10% ethanol blend.

Another factor is cargo and passengers. The tests are performed so that the total weight of all cargo and passengers equals only 300 pounds. If you have two adults in the front seat, and a car seat and two kids in the backseat, as well as cargo in the trunk, you’re probably carrying close to 400 or 450 pounds total of cargo, which can negatively affect your fuel economy.

But, the flip side is that if you keep your vehicle as free of cargo as possible, and keep a close eye on your driving habits, there is the possibility that you could get even better gas mileage than the EPA estimates. Keep in mind, part of the high-speed test involves a rapid acceleration from a stop all the way up to 80 miles per hour, and that’s not exactly something I make a habit of.

Why are these tests useful?

One common complaint about these tests is that, because of the strict conditions they are subject to, they do not accurately reflect real-world fuel economy. If these EPA fuel economy estimates don’t actually tell you what your fuel economy will be, what makes them useful?

The biggest benefit of these tests is the ability to do true apples-to-apples comparisons of different vehicles. Because every vehicle undergoes the exact same test under the exact same conditions (down to the exact humidity and temperature of the room that the test is performed in), you can easily compare one vehicle against another and know that the comparison is fair and accurate.

This is why Ford is so proud to have the best-in-class 2013 C-Max Hybrid (47-City/47-Highway/47-Combined), as well as the fuel efficient 2013 Escape (23-City/33-Highway/26-Combined, when equipped FWD, automatic transmission, 1.6L EcoBoost engine), and the other fuel efficient vehicles in the Ford line-up.

For more information on fuel economy ratings, please visit:

http://www.fueleconomy.gov/

For more information on how the EPA tests for fuel economy, and what it other things it tests for, visit:

http://www.epa.gov/otaq/standards/index.htm

For more information about the driving cycles that are tested, visit:

http://www.fueleconomy.gov/feg/fe_test_schedules.shtml

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