Almost anyone who drives, and especially those who drive for a living, have fuel economy on the brain. It’s probably on your mind right now, “Am I spending too much on fuel?” Part of it has to do with refueling costs, but others may also be wondering about their carbon dioxide emissions, which is directly related to fuel economy. Fuel economy, usually measured in miles per gallon (mpg) or gallons per 100 miles (gal/100mi), is proudly displayed on the window sticker of every single new car in the United States, by law as a matter of fact. Still, when you read the official EPA / DOT Fuel Economy and Environment (Environmental Protection Agency and Department of Transportation) window sticker, shown here, what exactly does it mean when it says that it gets 26 mpg or uses 3.8 gal/100mi?

• Simple – The sticker says that the EPA calculates this particular year, make, model, engine, and transmission to average 26 miles per gallon or use 3.8 gallons to drive 100 miles, which is based on the typical 15,000 miles per year the average American drives, which is calculated at around 40% highway driving and 60% city driving.

So that’s the basic answer, but what’s up with the small print? More importantly, what does “Your Mileage May Vary” (YMMV) mean for your fuel economy? Really, it should say, “Your Mileage Will Vary” (YMWV) because there are many factors involved that affect fuel economy, some for better, others for worse. Here are a few:

• Where – If you drive mostly highway miles, then your fuel economy should be higher as long as you don’t go too fast (See “Speed” next). Our example vehicle, a small SUV, is rated at 32 mpg highway. On the other hand, if you do mostly city driving and stop and go traffic, your fuel economy will be closer to 22 mpg, perhaps even lower, depending how much time you spend idling.
• Speed – Every vehicle has a fuel economy “sweet spot,” fast enough to get somewhere comfortably without burning excess fuel. For most vehicles, that sweet spot is around 55 mph, more or less, depending on the vehicle. Let’s say our little SUV gets 32 mpg at 55 mph. The EPA estimates that, for every 5 mph over the sweet spot, there’s a fuel economy penalty. At 60 mph, our SUV is getting 31 mpg, which doesn’t sound so bad. At 80 mph, she’s only getting 23 mpg, just barely better than city fuel economy!
• Cargo – Carrying around more weight than necessary, whether inside or outside the vehicle, also impacts your fuel economy in a bad way. The EPA estimates that every 100 lbs extra weight impacts your fuel economy by a percentage point, and that cargo carriers, even so-called “aerodynamic” rooftop carriers, can impact your fuel economy by up to 17%, even if they’re empty!

True, there have been cases of fuel economy being misrepresented by manufacturers, and the EPA is only able to test about 10% of the vehicles on the road. The EPA’s calculations are generally pretty accurate, otherwise, and give us a good idea of what to expect when comparing one vehicle with another. On the other hand, there is a lot that we can do to improve our own fuel economy, such as driving responsibly and keeping our vehicles and tires in good shape.