Road Trip Gas Cost: How to Work It Out and Split It
The short answer
Gas cost = (miles ÷ MPG) × price per gallon
Divide the distance by your car’s fuel economy to get gallons, then multiply by what a gallon costs. For a round trip, double the one-way miles first. To split it, divide by the number of people paying.
Example: a 470-mile drive each way is 940 miles round trip. At 32 MPG that burns 940 ÷ 32 = 29.4 gallons. At the U.S. average of $4.354 a gallon for regular in the week of October 5, 2026 (from the Energy Information Administration), the fuel costs $127.90, or about $32 each for four people.
The arithmetic is easy. Getting the three inputs right is where most estimates go wrong, so that is what most of this guide is about.
Step 1: Miles
Use the driving distance from a map app, not the straight-line distance, and include the side trips you already know about: the detour to the trailhead, the drive from the hotel to the venue and back each day. Local driving at the destination adds up quickly on a multi-day trip.
For a round trip, double the one-way figure. If the return route is different, add the two legs.
Step 2: MPG, the number that matters most
Fuel economy moves the answer more than anything else. Here is the same 940-mile round trip at $4.354 a gallon:
| Fuel economy | Gallons | Fuel cost |
|---|---|---|
| 24 MPG | 39.2 | $170.53 |
| 28 MPG | 33.6 | $146.17 |
| 32 MPG | 29.4 | $127.90 |
| 40 MPG | 23.5 | $102.32 |
| 50 MPG | 18.8 | $81.86 |
Going from 24 to 32 MPG saves about $43 on one trip. So it is worth choosing the right figure:
- Best: your own logged MPG on a recent highway tank. Fill up, reset the trip meter, drive, fill up again, and divide miles by gallons. 312 miles on 11.4 gallons is 27.4 MPG. The gas mileage calculator does this and keeps a log in your browser so you can see your average.
- Next best: the EPA highway rating for your exact year, model and engine on fueleconomy.gov.
Then adjust for how you will actually drive. fueleconomy.gov’s driving tips give some useful sizes:
- Aggressive driving (speeding, hard acceleration and braking) can lower gas mileage by roughly 15% to 30% at highway speeds.
- A large, blunt roof-top cargo box can cut fuel economy by around 6% to 17% on the highway and 10% to 25% at Interstate speeds.
- An extra 100 pounds in the vehicle could reduce MPG by about 1%.
- Idling can use a quarter to a half gallon of fuel per hour, depending on engine size and air conditioning use.
Take the roof box. If it knocks 10% off your 32 MPG, you are at 28.8 MPG: 940 ÷ 28.8 = 32.6 gallons, and the trip costs $142.11 instead of $127.90. The box costs about $14 in gas on this one trip.
Step 3: The price per gallon
Gas prices vary a lot by region, and the national average can mislead you on a long trip. The EIA publishes weekly averages for regular gasoline. For the week of October 5, 2026:
| Area | Regular, $ per gallon | 940-mile trip at 32 MPG | Each of 4 |
|---|---|---|---|
| Gulf Coast | $3.911 | $114.89 | $28.72 |
| Midwest | $4.109 | $120.70 | $30.18 |
| U.S. average | $4.354 | $127.90 | $31.98 |
| Rocky Mountain | $4.417 | $129.75 | $32.44 |
| West Coast excluding California | $5.116 | $150.28 | $37.57 |
| California | $6.227 | $182.92 | $45.73 |
The same trip costs $68 more in California than on the Gulf Coast. If your route crosses regions, a rough fix is to price each leg at the price where you will fill up. Prices change every week, so check the EIA update or local pump prices close to your departure date.
Worked example: four friends, two ways to split
Four friends drive 470 miles each way in a car that gets 32 MPG on the highway, filling up around the U.S. average price.
- Miles: 470 × 2 = 940.
- Gallons: 940 ÷ 32 = 29.375.
- Cost: 29.375 × $4.354 = $127.90.
- Cost per mile: $4.354 ÷ 32 = 13.6 cents.
Everyone pays, driver included: $127.90 ÷ 4 = $31.98 each, rounding up so the tank is covered.
Passengers pay, driver supplies the car: $127.90 ÷ 3 = $42.63 each. Some groups do this because the driver is already absorbing wear on the car. There is no single right answer; agree on it before you leave rather than at the last pump.
If one person leaves halfway or joins for one leg only, split each leg separately: the cost per mile (13.6 cents here) times that leg’s miles, divided by the people in the car for that leg.
What fuel cost leaves out
Gas is only part of what a road trip costs. Tolls, parking and any overnight stops are separate. So is wear on the car: tires, oil and depreciation. For a sense of scale, the IRS business mileage rate, which taxpayers can use instead of adding up actual car expenses such as depreciation, registration and gas, is 76 cents a mile for driving from July 1, 2026. Fuel alone is 13.6 cents a mile in this example. That gap is one reason some groups give the driver a bit more than an even share of the gas.
Common mistakes
- Using the one-way distance for a round trip. Easy to do in a hurry, and it halves the estimate.
- Using the combined MPG for a highway trip, or the highway MPG with a loaded roof box. Pick the figure that matches the driving.
- Pricing a cross-country trip at your home price. Use the price where you will actually fill up.
- Forgetting driving at the destination. A week of local trips can add a full tank.
- Rounding gallons down. You buy fuel in tankfuls, not decimals. Budget a little over.
Quick checklist
- Driving miles, both ways, plus known side trips.
- Your real highway MPG, minus a margin for cargo, roof boxes or high speeds.
- The pump price where you will fill up.
- Gallons × price, then divide by the people paying.
The fuel cost calculator runs those steps for one-way or round trips, splits the cost between passengers, and gives you a link to share the numbers with the group.