“Your mileage may vary” is one of the oldest proverbs in the automotive rulebook. But even though the EPA is constantly looking for ways to improve its testing procedures and rating calculations, “YMMV” still applies to today’s EPA fuel-economy ratings—fueleconomy.gov even has an entire section titled “Your Mileage Will Still Vary.” So we devised our own test procedure in an attempt to better replicate how most people drive on the highway, beyond the constraints and complicated math of the government tests. Our test entails a 200-mile out-and-back loop on Michigan’s I-94. Drivers are instructed to maintain a GPS-verified 75 mph, using the cruise control as much as possible, and to run the air conditioning at 72 degrees if the vehicle has automatic climate control, or at a moderate setting if it does not.

After testing more than 100 vehicles through December 2016, we tabulated the results to find outliers. Here are the biggest underachievers, ranked in order of the percentage of difference between our real-world test result and the EPA-rated highway mpg number. To see the biggest overachievers, check out this list.