Mileage will always vary.

That’s the disclaimer that comes with every car. The statement is certainly accurate. Two people driving the same car on the same route will likely get different fuel economy numbers because no two people drive exactly the same.

The April announcement from Mitsubishi Motors that it rigged fuel economy tests on 625,000 vehicles dating back to 2013 is just another blow against the auto industry that has been marred by fuel-fixing scandals. (None of those vehicles were sold in the U.S.)

How does this even happen? For starters, carmakers do almost all of the testing on fuel economy and report those findings to government regulators like the Environmental Protection Agency and the National Highway Traffic Safety Administration. Occasionally, the government will test a vehicle and verify those numbers, but there are too many vehicles and not enough time or money for it to test every vehicle. Instead, carmakers use a highly-calibrated machines to test their vehicles, allowing every carmaker to conduct exactly the same test to their vehicles and provide data that should be comparable to every other vehicle.

Unless, of course, the carmaker cheats. Mitsubishi is the only the latest carmaker to get caught gaming the system. Last year, Volkswagen admitted to using slick computer programing to allow millions of its diesel-powered vehicles since 2009 to trick the governmental testing around the world and provide better mileage. The repercussions in the VW case remain to be seen but will likely cost the company billions of dollars in fines and repairs. In 2014, Hyundai Motor and Kia Motors paid a $350 million U.S. fine because it exaggerated its mileage numbers in a number of cars. Ford Motor Co. two years ago had to restate the mileage figures on six vehicles because of bad calculations.

In most cases, carmakers refund owners a small amount of money – typically the difference between real mileage and predicted mileage – and pay a fine to the government. Then, the issue ends.

So do those mileage numbers mean anything at all? Sometimes, yes.

First, the overall, the vast majority of vehicles have been tested fairly and the numbers accurately reflect a vehicle’s mileage in stop-and-start city traffic and highway cruising. The numbers are best used for comparison of similar vehicles. If you’re looking at a compact sedan and see the Honda Civic gets 31 mpg in the city and 42 mpg on the highway, and the Chevrolet Cruze gets 30 mpg city / 42 mpg highway, you can be fairly certain these similar sedans will get about the same mileage if you were driving them.

Of course the owner’s driving habits play the most important role in good fuel economy. If you have a lead foot, your fuel economy will go down. If you drive mostly in the city you are going to have fuel economy numbers closer to 30 mpg than to 40 mpg in either the Civic or the Cruze.  Smooth acceleration, going the speed limit and maintaining the correct tire pressure will do more to help your fuel economy than anything else.

However, not all fuel economy numbers are the same. The current car market now includes a number of alternative powertrains such as hybrids, plug-in hybrids and electric vehicles that offer fuel economy numbers that include MPGe.

Hybrids are vehicles that use an electric motor to assist the car’s forward propulsion. They will use the electric motor only in typically slow stop-and-go traffic. This is why they have very high city mileage compared to its highway fuel economy. The iconic Toyota Prius gets 58 mpg in the city and 53 mpg on the highway.  Earlier hybrids had even better mileage because they originally used the same test as regular gas-only vehicles, which scientists quickly realized, gave inaccurate results.

Plug-in hybrids operate like a regular hybrid but have the added benefit of a larger battery pack that can be charged and drive the car for extended range. (They are often referred to as extended range hybrid vehicles.) The most well-known, the Chevrolet Volt can drive 53 miles on its batteries before its gas engine kicks in to create power for the motors. Then it gets 42 mpg, but even that number does not match up directly to a gas-only vehicle.

Pure electric vehicles such as the Tesla Model S or the Nissan Leaf offer an MPGe number, which stands for miles per gallon equivalent. The Model S hits 92 MPGe and the Leaf gets 114 MPGe. These numbers are made to compare efficiency between electric vehicles but fail to tell the full story. The Model S has a range of up to 250 miles, while the Leaf’s range is up to 107 miles.

Of course, mileage will always vary

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Free Vin Lookup

Vin Lookup

The Vehicle Identification Number is located in many different places on a car. It is very common to appear on the dashboard and/or in the drivers side door sticker. You can do a free VIN look up to check vehicle problems on the button above.