How to Calculate Gas Mileage for a Trip

Nothing says "carefree" like the great American road trip – until you run out of gas or find yourself waiting tables to pay for that next tank of fuel. Avoid such nasty surprises by working out your vehicle's gas mileage beforehand and then using that information to budget mileage – and gas money – for your trip.

Almost every modern vehicle has an onboard computer that will calculate your gas mileage for you; just toggle to that part of the display, reset the computer, then drive normally for at least half a tank of gas. If there's something wrong with your trip computer or your car doesn't have one, you can calculate your mileage manually: Fill your gas tank completely, reset your trip odometer (or write down the mileage from your master odometer) then, as before, drive normally for at least a half-tank of gas.

When you refill your car again, take note of how much gas it took, along with how many miles you drove. Divide the number of miles driven by the gallons of gas you put in the car; the result is your car's typical miles per gallon. For example, if you drove 200 miles on 8 gallons of gas, your car averaged 200/8 = 25 miles per gallon.

Now that you have an idea of your vehicle's average gas mileage, it's time to plan the route for your trip. Start by choosing the main attractions you'll visit and how long you're willing to spend in your car. Some people can blitz through 10 hours of driving in a day, especially if they're switching off with another driver; for others, six hours in a car is more than enough. The point of a road trip is fun, so make sure you choose something that you – and everyone else in the car – can enjoy. Factor in time for rest stops, meal breaks, photo opportunities and exploring attractions along the way.

Once you've settled on an average drive time, you have a useful way to compare different routes between the attractions you've chosen. Some of the factors to consider include the type of scenery you'll see – back roads tend to be slower but more scenic, while interstate highways can help you zip through a region you're not particularly interested in seeing, if they're not notorious for traffic jams – and where you can find rest stops, lodgings or restaurants. Take into account your total gas mileage, of course.

Once you've selected your route, you can use any number of online mapping tools to tote up the mileage for your trip. Divide this by your vehicle's average miles per gallon to get a rough idea of how many gallons of gas you'll need to buy during your trip. With that said, remember that gas prices can vary enormously by region, then fluctuate even more according to supply and demand. You can use tools like Gas Buddy to estimate your gas cost in a given part of the country, but even then, make sure you add in at least a 10 percent fudge factor to include things like wrong turns, detours, sudden surges in gas prices, or decreases in fuel efficiency from sitting in traffic jams.

Tip

Most online trip planners can also help you plot your fill-up points. But in some rural areas, there may be very long stretches between gas stations, or the gas stations may only be open during very limited hours. If you're road-tripping through remote areas, it's always a good idea to hedge your bets by carrying a small can of extra gas. Even if you don't need it yourself, you might be able to help out someone else in distress.