How much gas for a road trip, and what the fuel number leaves out

The fuel number in three inputs

Fuel cost is distance divided by consumption, multiplied by the pump price. Getting it wrong is almost always one of these three, not the arithmetic:

• Distance. The driving distance is not the map distance between two dots. Across the United States, interstate routing typically adds 15–30 % to the straight line, because the road goes around the Appalachians and the Rockies rather than over them. Budget for the road figure, and remember to double it if you are coming home.
• Consumption. Use the mpg the car actually returns on the interstate, not the window sticker. Loaded with four people, luggage and a roof box at 75 mph, a car that is rated 32 will often do 26–28. A roof box alone costs around a tenth of the tank.
• Price. State taxes move the pump price by more than a dollar between, say, Texas and California, so a national average is a starting point and not a quote. The tool prefills one and expects you to overwrite it with what you saw on the road.

One more trap for anyone comparing notes across borders: the US gallon is about 17 % smaller than the imperial gallon. The same car reads 34 mpg in Ohio and 41 mpg in Ireland. Never mix the two in one calculation.

What usually beats the fuel bill

On anything longer than a day’s driving, fuel stops being the headline:

• Rooms. Two nights at typical interstate rates outweigh a full tank on most cross-country routes. This is the line that decides whether a trip is cheaper than flying.
• Food. Four people, three days, restaurant stops — this quietly matches the fuel line and is the easiest one to halve with a cooler.
• Tolls. In the US these are billed by plate or transponder and vary by road, so the operator’s own figure is the only reliable one. In Europe the shape is different: distance-based tolls in Italy, France, Spain and Portugal, and vignettes in Austria, Switzerland, Slovenia, Czechia, Hungary, Slovakia and Romania that are mandatory from the first kilometre of motorway — driving without one is a fine, not a bill, and the rental company will pass it on with an administration charge.

Split it per person before you decide

The reason to total a road trip properly is the comparison with flying, and that comparison is per person. Fuel and tolls divide by everyone in the car; rooms divide by the number of rooms; food does not divide at all. Four people sharing one car is what makes driving win, and the same trip alone is usually the more expensive option.

The calculator below does the split, keeps each country’s tolls in its own currency rather than inventing an exchange rate, and gives you a card you can send to whoever is coming with you.