How to Calculate Cost of Gas for a Road Trip

Couple road tripping at sunset
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You're thinking a road trip seems like a cheaper way to travel across the United States than flying. But is it? Finding the answer involves the current price of gasoline, the miles per gallon your vehicle gets on the highway, and the miles you intend to drive on your trip. This might sound a little complicated, but it's really pretty simple. You can get the bottom line on this in four easy steps.

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Calculate Gas Mileage (MPG)

The first thing you need to do is calculate the miles per gallon (mpg) your vehicle gets. You can do this yourself with a little simple arithmetic or you can use a mileage calculator. If you want to do it yourself, the next time you fill up your car, note your odometer reading or set your trip odometer to zero by pushing in the little knob underneath the odometer or using your computerized console.

Then just drive normally until it's time to fill up again and note the odometer reading or the miles on the trip odometer when you fill it up again. Subtract the first odometer reading from the second to give you the number of miles you've driven. Or if you set the trip odometer to zero when you filled up the last time, that number is the number of miles you have driven on that tank of gas.

Divide that number of miles by the number of gallons you just purchased on your second visit to the gas station, and this will give you your miles per gallon. Note this number down.

Research Your Trip Distance

Next, you need to calculate the total distance you'll be driving on your road trip. For this, you can use AAA or Google maps. Enter in your start and finish points, along with any stops along the way, check that the route it's plotting out is likely to be the one you'll be taking, and then make a note of the number of miles it says your trip will cover.

If you're planning a several day-, week-, or month-long road trip, you won't be able to calculate the exact distance because of side trips and random detours, so it's best to make a guess based on your planning so far. If in doubt, add in some side trips to your total, so if you decide to skip out on them, you'll be spending less money than you budgeted for. Jot down the total distance you'll be driving next to your figure for miles per gallon.

Find Out the Current Price of Gas

For the third step, you need to know the current price of gas to keep your total figure as accurate as possible. It's not possible to know the exact cost of a gallon of gas at any given time in any given location, but knowing the current price will give you a dependable approximation of the total cost of fuel for your trip. Use AAA to find the average national gas price. Jot down the amount given at the top of the page as your third figure.

Add Up Your Trip Cost

Now you have all the information you need to figure out how much the gas for your road trip will cost. Take the mileage of the total distance of your trip and divide it by your miles per gallon to get the number of gallons of gas you will need on your trip. Then multiply that figure by the current price of gas, and the result is the cost of gas for your road trip.

As an example, say you drove 200 miles after you filled your car up, set your trip odometer to zero, and had to return to fill up again. On your return to the gas station, you topped up your tank with 10 gallons of gas. Your miles per gallon would then be 200 divided by 10, which is 20 mpg. You are planning on driving 850 miles on your road trip. The average price of gas was $2.34 at the time you checked.

To calculate the total amount of money you need to budget for your road trip, you divide 850 by 20 to get the number of gallons of gas you will need: 42.5. You then multiply 42.5 by $2.34, which gives you $99.45 as the total cost of gas for your road trip. 

Factor in All Car Travel Costs

The cost of gas is only one aspect of the expense of a road trip. You'll also need to factor in lodging, meals, maps, road tolls, and other car-related costs. If you're traveling with a companion or group of friends, you can divvy up the cost of gas, so your transportation fee will be even less per person.

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