Gas is usually the biggest road trip expense you can actually calculate before you leave. Here’s the math, and the things the math leaves out.
Road notes
- Fuel cost is roughly total miles divided by your car’s real-world mpg, multiplied by the price per gallon.
- Highway speed, a loaded car, roof boxes, mountains and air conditioning all reduce mileage; plan with a cushion.
- Fuel prices vary a lot between states and between stations along the same highway.
- Tolls, parking, food and lodging often add up to more than fuel; budget them separately.
You can’t predict every expense on a road trip, but fuel comes close. Run the numbers before you go, add a cushion, and you’ll have a budget that holds up when the price board at the next exit is higher than expected.
The basic formula
Fuel cost = (total miles ÷ real-world mpg) × price per gallon
A worked example: a 2,400-mile round trip in a car that averages 28 mpg on the highway needs about 86 gallons. At $3.50 a gallon, that’s about $300. Add 10 to 15% for detours, idling and heavier loads, and budget around $330 to $345.
For an EV, swap in miles per kWh and the price per kWh at the chargers you’ll use, which can differ a lot from home charging.
What lowers your mileage
| Factor | Why it matters |
|---|---|
| Highway speed | Mileage usually falls quickly above about 50 mph |
| A loaded car and roof box | Weight and drag both cost fuel |
| Mountains | Climbing uses more fuel than descending saves |
| Air conditioning and extreme heat | Extra load on the engine |
| Low tire pressure | Increases rolling resistance |
Checking tire pressure before you leave is the cheapest fuel saving there is; our pre-trip checkup covers it.
Finding better prices
- Use a price app to compare stations ahead on your route.
- Watch state lines; fuel taxes vary, so prices can change at the border.
- Avoid the last station before a long empty stretch if you can fill up earlier for less, but never gamble with an empty tank in remote areas.
- Check whether a warehouse club or grocery loyalty program you already have offers fuel discounts.
The rest of the budget
Fuel is only one line. A realistic budget also includes:
- Tolls, which can be significant on some routes; see our guide to tolls on road trips.
- Lodging, usually the largest cost after fuel.
- Food, cheaper with a cooler and grocery stops.
- Parking in cities and at popular sights.
- Park entry and activity fees.
- A repair and emergency cushion.
Road-tested tip
Keep a running total in your phone’s notes app as you go. Seeing the numbers each evening makes it much easier to stay on budget for the second half of the trip.
Questions from the road
Q: Where can I find my car’s real mpg?
A: Your car’s trip computer is a good start, and the EPA ratings at fueleconomy.gov give a baseline. Real highway mileage is often lower when the car is loaded and moving fast.
Q: Is it cheaper to fill up before crossing a state line?
A: Sometimes. Fuel taxes differ by state, so prices can jump at a border. Price-comparison apps show what’s ahead on your route.
Q: Should I use premium fuel on a road trip?
A: Only if your owner’s manual requires it. If premium is merely recommended, regular is usually fine; check the manual for your car.
Road rules, fees, closures and border requirements change. Check with the state transportation department, park, rental company or border agency before you travel. This guide is general information.
