How to Calculate Road Trip Fuel Cost Accurately

The difference between a skimpy fuel budget and a more substantial one could be a skipped detour off the road, a meal at the campground, or a dash of grace in the plan for that pie shop two exits down the road. To get a more realistic sense of fuel cost for a road trip, a traveler needs more than just a mileage figure. A route, the car’s fuel economy and some allowance for the ebb and flow of the road are necessary.

Fundamentally, to figure fuel cost for a trip, three numbers are required.

The road trip fuel cost formula

Let’s say a driver wants to drive 1,200 miles in a car with 28 mpg. And let’s say gas is expected to be $3.60 a gallon. At a rough estimate, fuel will cost $155. Then build in some allowance for price fluctuations, side roads, and traffic. $175 would be more realistic.

With an electric vehicle, the same calculation applies. Let’s say an EV does 3.2 miles per kWh on a 900-mile trip and that on average the driver pays $0.42 per kWh. The vehicle will need about 281 kWh of electricity. That would bring the expected charge cost to about $118. Depending on charging at home before leaving, weather, price on the charging network and use of fast public charging, this could well vary.

Trip miles ÷ miles per gallon × average fuel price = estimated fuel cost

It’s easy math. But better planning of a trip starts with picking the right inputs.

Start with the route you will actually drive

Miles for a trip should be more than just the distance from home to a destination. They should include, for example, the loop up through Sedona, driving from the gate of a national park to a lodge, a trip into the campground for supplies, and that time when everyone wants to check out the odd and wonderful roadside attraction.

On a one-way trip, use the miles on the route. On a round trip, only double them if the same route will be taken back.

1,200 ÷ 28 = 42.9 gallons

42.9 × $3.60 = $154.44

Often, the best trips will take a different route home. Maybe up the Blue Ridge Parkway one way and Interstate 81 the other; or a drive along the coast one way and inland the other. A trip with six overnight stops can easily be hundreds, even hundreds of miles longer than a direct route. That’s especially true in rural areas, national parks, mountains and along roads that snake with the terrain rather than in a straight line.

Before planning a trip, think through each of the key stops. A traveler can then estimate fuel use for the trip desired. A basic map will assume a different route.

A tool like roadtrippers can take a list of saved places and build a route. That can help gauge how much more mileage and cost each interesting detour will add.

Trip miles ÷ miles per kWh × average charging cost per kWh = estimated charging cost

Use real-world MPG, not the best number on the window sticker

A useful starting point is the combined fuel economy rating for a car. But it isn’t necessarily accurate. Depending on passengers, gear, a rooftop box, coolers or bikes, a car may get much less than that. And in freezing weather, heat, wind, hills or stop-and-go driving, performance could be even worse.

Best of all, go back to recent driving data. After filling up, take a long drive in the car. At the next fill-up, divide the miles driven by the gallons put in. Repeat this two or three times if possible. Take an average of the results. That will be a more realistic basis for planning than an overly optimistic number.

If fuel use hasn’t been tracked lately, err on the conservative side. Let’s say there is a sedan with a 32 mpg rating. A traveler could plan for 28 to 30 mpg on a trip with people, gear and coolers. A truck or SUV with a 20 mpg rating might well do 17-18 mpg under similar circumstances.

A vehicle could well do better than that, but to preserve fun money in a trip, it makes sense to plan for somewhat less.

Towing and RV travel change the equation

For tow vehicles and RVs, actual mileage matters more than best-case MPG. A pickup pulling a travel trailer through the Rockies, for example, or a motorhome moving against a headwind over West Texas, will use far more fuel than it would on a local flat highway.

Rather than relying on best-case MPG, pick a figure that reflects treks of similar length and load on your rig. If you can’t find such data, talk to other owners of your kind of rig, and pick the lower end of their range.

10 mpg is not as exciting as 12, but it is easier to be pleasantly surprised at the pump than to drain your camping budget before the first park entrance.

Don’t forget idling. Long waits at border crossings, hot rest stops, climate control at night and running the generator all use fuel, but don’t move you down the road.

Pick a realistic average fuel price

On a long trip in your RV, it would be a good idea to build in some extra fuel, rather than squeezing those hours into your MPG calculation.

Fuel prices vary across states, seasons and depending on whether you’re near a big highway or a recreation area. If you’re driving cross-country in your RV, you wouldn’t just want to base your fuel costs on your local station price. Rather, you could pick an average price for the regions you’ll be driving in.

For most trips, it’s more useful to pick an average price of fuel and build in some extra than to focus on the exact price at each station.

Let’s say you think fuel will run $3.40-$3.80 per gallon on your trip. At $3.70, you’d be well in the middle of that range. If you’re driving in an area where fuel is more expensive (say, the mountains or parts of California), add a bit more to your average price.

Drivers of diesel, regular gas and premium gas all should use different fuel price assumptions.

Add a fuel buffer for the miles you did not plan

Diesel prices are often quite different from regular gas prices, and in some places are much higher. And vehicles that call for premium gas should use premium gas prices, even if from time to time they can pick up a bargain station.

A fuel estimate is best used as part of your planning. It doesn’t have to be 100% accurate. Give yourself 10% for a point-to-point road trip and 15-20% for an adventure with mountain passes, city driving, park roads and/or spur-of-the-moment exploration.

This buffer would pick up more than just detours. It could pick up a closed scenic road, a construction detour, a trip to find a trailhead or a drive into town to refill the ice chest at your campsite. And it could pick up an extra night under the stars, a museum ticket, or a meal in town.

More importantly, it could pick up a tank of gas at a price that seems right at the time, instead of one you might regret later at the next pump. And even if you don’t need it, this buffer isn’t money down the drain. It could pick up a day on the road in place of a costly attraction, a stop at a museum, a meal or more time in your campsite.

Here is what a fuller estimate might look like for a 1,200-mile SUV trip:

Route mileage: 1,200 miles
Realistic fuel economy: 18 mpg
Expected average gas price: $3.80 per gallon
Base cost: 1,200 ÷ 18 × $3.80 = $253.33
15% exploration buffer: about $38
Planning fuel budget: about $291

Often the biggest variable cost on a road trip is fuel. But so too are food, lodging, park passes, tolls, parking and maintenance of your vehicle.

A longer route doesn’t necessarily cost more than a shorter one. A scenic longer drive could save you money on downtown lodging, could include more free stops in nature, could take the place of an expensive attraction with a day on the road, and could simply be more fun.

But often the shortest route isn’t the cheapest. Speed cuts into fuel efficiency. Toll roads cost money. And in cities, a short route may well use more fuel than a slightly longer drive on the highway.

None of this is right or wrong. What’s important is your priority: time, scenery, low fuel consumption or visits to specific places. And you could work out the cost of a route to suit that priority.

If you’re traveling as a group, before you set off, agree among yourselves how to split up your fuel costs.

Factor fuel costs into the bigger trip budget

Often, fuel is a huge variable cost on a road trip. But it has to fit with food, park passes, lodging, campgrounds, tolls, parking and vehicle maintenance. A longer route doesn’t necessarily cost more. An extended scenic trip might save you expensive downtown lodging, give you more free outdoor adventure, or in place of a costly attraction, could well give you a great day on the road.

But often the quickest route isn’t the cheapest. At high speed, fuel economy drops; toll roads add up; and in the city, a short route may well use more fuel than a longer highway route. None of these options is better than another. Depending on whether you value time, scenery, low fuel use or access to specific places, pick a route that suits you. And price it.

If you’re traveling as a group, work out (between yourselves) how much fuel each person will need. Friends in one car could simply split their gas receipts. In a family trip, think of fuel as a household cost. If you’re driving separate vehicles in a caravan, each of you should work out your own fuel needs. A hybrid crossover, towing truck and camper van will need different amounts of fuel.

Check your estimate as you travel

As you start your trip, in the first two fill-ups you’ll get a sense of whether your plan is working. Compare your estimate of gallons, miles and price with what actually happens.

If mileage is worse than you expected, think about cutting back on the budget for fuel over the rest of your trip. Perhaps you could drive a bit more slowly, run more errands at each stop, or pick a campground closer to where you’ll be the next night.

If you do better than expected, don’t assume this margin will carry you through the next mountain range. Traffic, elevation, wind and a heavy load can flip the equation quickly.

Best of all, a fuel budget should allow for surprises on the road. Think through your numbers, but leave some room in your budget for when you see a sign painted on a roadside that looks worth checking out.