How to Calculate Miles per Gallon
By Fred Decker; Updated June 08, 2017Calculating gas mileage for your ultimate road trip
Road trips are one of the great things in life. They're a way to see the country, spend time with those you love, or just get away from all the stresses of daily life. That doesn't mean you get to forget about practicality altogether, of course. For one thing, you'll need to know what you can expect to spend on gas while you're soaking up the scenery. A few tips can help you make a realistic estimate of your mileage and gas budget.
The basic mileage calculations
The basic calculation is pretty simple. You need to know how far you'll be driving, what mileage your car gets, and what the price of gas will be along the way. To get the number of gallons you'll burn, divide the length of your trip by the miles per gallon you can expect from your car. Multiply that by the average cost of gas, and you'll have a ballpark estimate of your fuel costs for the trip.
You can get the first number from your GPS device, smartphone app or an online mapping service, so that part's easy. To work out your gas mileage, top up the tank, drive for a couple of hours, and top it up again. When you divide your miles traveled by the gallons used, you'll have a pretty accurate number. If you don't have time for that exercise, look that typical mileage for your make and model on the U.S. Department of Energy's fueleconomy.gov website. Finally, the Energy Information Administration provides average gas and diesel prices on its website, by region or major population center.
It's not always open highway
Ideally that's all the work you'd need to do, but in real life, it's not that straightforward. Unless your home and destination are just off the open highway and you can avoid cities along the way, you're going to need to account for city driving. For non-hybrid cars, that means a big drop in mileage, and you'll need to account for that extra consumption. Look at how much of the drive will be spent in urban areas or crowded corridors such as those surrounding Los Angeles or New York. Allow for extra traffic if you're passing through at rush hour, or at either end of a long weekend. Work out the rough percentages for your trip – 70 percent highway to 30 percent city, for example – and then calculate your highway and city mileage separately. Add those numbers together for a more realistic figure.
Your car, right now, isn't "the book"
If you're using your car's "book" mileage from fueleconomy.gov or some other source to calculate mileage, it's important to recognize that those figures might not be accurate for your car. Depending on its age and how well it's been maintained, your real-world mileage might fall well short; especially if you're heavily loaded or pulling a trailer. Fueleconomy.gov estimates that every 100 pounds of load costs you about 1 percent of your potential mileage, and roof-mounted carriers can drop your mileage by up to 25 percent. Air conditioning cuts your mileage by 5 percent or more, as well. For the most accurate possible estimate of your mileage, load the car the way you'll drive it on your trip and repeat your drive-and-fill-up experiment.
Geography matters, too
Even then, your mileage might not be exactly what you've calculated. Personal driving style factors into this – impatient drivers tend to burn more gas – and so does geography. If you plan to spend hours barreling down the relatively flat interstates of the Great Plains in a lightly loaded car, your mileage will be great. If you're hauling a small vacation trailer up and down the Rockies, that's a whole different story. There is no easy way to allow for uphill driving, but if you'll do a lot of it, you should plan on a few extra gallons.
Turning to technology
You can simplify this, if you want to take advantage of technology. For a start, you might use a trip calculator, such as that on fueleconomy.gov, to calculate a rough fuel cost for your trip based on the car you drive and your start and end points. Your GPS device, or GPS app, often offers the option to pick a route for best mileage rather than for speed. Smartphone apps such as Fuelly, Fuel Buddy and MileIQ can all help you painlessly track and improve your car's real-world mileage, and the GasBuddy app can help you find the best gas prices wherever you are.