How Do We Measure Fuel Economy?

What MPG Really Means

When you’re shopping for a new compact SUV, there are plenty of metrics that you’ll investigate: horsepower, cabin space, and MPG, to name a few.

But what does it actually mean when you say that the Chevrolet Trax earns a class-leading 26 city and 31 highway MPG? Even though we often feel like we know what this means, it’s worth exploring.

Where Do Fuel Economy Numbers Come From?

Truth be told, most fuel economy numbers come straight from the manufacturer. While the EPA oversees the issue of fuel efficiency testing, it just doesn’t have the resources to test every single car made each year. The EPA does create all of the complex rules and regulations around fuel economy testing, but their lack of testing is why most cars are said to have “an EPA-estimated fuel efficiency…”

Even though the EPA doesn’t personally test every car, it does audit about 15% of vehicles to check for accuracy. Since most auto manufacturers have entire departments devoted to working with EPA testing regulations, their numbers are typically accurate. Of course, getting caught falsifying test results is huge fines that just aren’t worth the risk.

Measuring Fuel Economy

You might think that Chevrolet or the EPA simply drives a Chevrolet Trax around a track a few times while measuring how much fuel is used, right? Unfortunately, measuring fuel economy is much more involved. To get accurate numbers, tests need to be completed in a controlled, repeatable environment.

Fuel economy testing typically takes place on something called a chassis dynamometer, which is essentially a treadmill for cars. That makes it easy to measure the distance traveled. Fuel usage is then measured by large tubes that go directly into the car’s exhaust pipe. Testers use the chemical makeup of the exhaust to determine how much fuel is used.

To make all tests consistent, the EPA requires something called Drive Cycles. These cycles are pre-determined vehicle speeds for a specific time. Different cycles give us the numbers for the city and for the highway. A city test is meant to simulate the stop and go traffic you experience on surface streets while highway cycles stay at more consistent speeds.

There are other, more complicated tests as well. The EPA uses in-depth formulas that help account for wind, tire pressure, fuel types, road surfaces, and more. The only other number that people tend to care about is the combined MPG. This is simply a weighted average of city and highway: 55% of the former and 45% the latter.

The Big Misconception

While using consistent tests across the auto industry should produce accurate results, it’s not always that easy. The reality is that you don’t drive a pristine Chevrolet Trax on a treadmill. You drive a car weighed down with cargo, on unpaved roads, with used tires. That’s why the car you bought advertises 40 MPG, but you’re lucky to get 33.

The EPA knows that its tests aren’t perfect. The EPA webpage has an entire section that describes that many, many elements that can alter the final results: weather conditions, driving style, vehicle condition, engine age, and more.

The Bottom Line

Now that you understand how fuel economy is determined and the inherent flaws with the tests, you can take this metric for what it’s worth. Yes, it’s worth paying attention to the EPA-estimated fuel efficiency, but you may not want to count on that exact number with every tank. Still, if you’re buying a Chevrolet Trax from your local Chevy dealer, you know you’re getting a ride that will give you want you want, all while saving you lots of money at the gas pump.