How Much Gas will I Use?By Fred Decker; Updated June 08, 2017
Successful road trip planning: Calculating how much gas I need
Working out the cost of some trips is a breeze: Once you've got the all-in price of your ticket, including fees and taxes, you know how much a flight or a train will cost. Driving is a little more complicated, because apart from meals and lodgings, you'll have to account for your gas consumption. That should be a simple calculation, and it is in theory – but some practical real-world complications might throw off your math.
The three main variables
The basic math involves three main bits of information: the gas mileage of your car, the price of gas along your route, and the distance you're traveling.
Unless your car is brand new, you should have a pretty good grip on the mileage you're getting. Even if you've never done the math, you probably know how many miles you'll get on a tank, and how much gas it takes to fill up. Fuel prices are available by region from the Bureau of Labor Statistics and the Energy Information Administration, or you can get up-to-date pricing for specific locations through various apps and websites.
Finally, you can get the distance between your two points from paper maps, a mapping program, or your GPS. From there, divide your travel miles by your miles per gallon, and then multiply by the price per gallon. That's simple, but it's not the whole story.
City vs. highway as portions of your trip
Your fuel economy will vary pretty widely depending how much of your trip is spent on the open highway, and how much getting to and from that highway. If you're driving from Los Angeles to San Francisco, for example, you might spend a lot of the day's drive merely inching toward your freeway ramp. Then there's the question of stops along the way: Are you going to whip into Bakersfield and grab some Basque food for lunch? Often you can miss the worst of the traffic by adjusting your departure and arrival times, but you'll still need to account for it.
Consider the topography and other variables
Other variables can have a distinct impact, too, such as the landscape you'll drive across. Crossing the plains of the Midwest is great for your fuel economy, but lumbering up mountainsides in the Rockies or the Sierras is a different story, especially if your car is heavily loaded. If you're pulling a trailer, it will cut into your gas mileage even on level ground, and hurt it significantly in the mountains. Another important variable is more personal: Patient, low-key drivers tend to get better mileage than tightly wound, pass-everyone drivers. It's something to think about on long trips, when you share driving duties.
Explore alternative routes
Taking the road less traveled can have a big impact on your fuel economy. Choosing alternative routes to avoid traffic, to find lower gas prices or to shun mountain passes for level ground as much as possible can all help keep your costs down. This used to require a sometimes-impractical level of local knowledge, but in the modern era, your GPS or your favorite mapping site can usually offer up those alternatives in a heartbeat. You might still decide to do the mountains, but it will be an informed decision, and you'll know the difference in cost.
Try a trip cost calculator
One way to streamline the process is through a trip cost calculator, such as the one at the U.S. Department of Energy's fueleconomy.gov website. You'll follow the prompts to select the make and model of your vehicle, your starting point and destination – and any side stops along the way – and the percentage of your trip that is city driving. The site fills in the "book" fuel economy of your car and current gas prices, and provides you with a step-by-step route. Other calculators may have additional features.