How to figure gas cost for a road trip
Three numbers decide what a drive costs in fuel: how far you're going, how many miles your car gets per gallon, and what gas costs. Put them together and you've got the bill before you leave the driveway.
The formula
Gas for a trip is just gallons used × price per gallon, and gallons used is distance ÷ MPG:
cost = (miles ÷ MPG) × price per gallon
A 600-mile trip in a car that gets 30 MPG, with gas at $3.50, is
(600 ÷ 30) × 3.50 = $70. For a round trip, double the distance (or the
result) — that 600-mile drive there and back is $140.
Worked example: family road trip, Baltimore to Charleston
Let's say you're driving from Baltimore, MD to Charleston, SC — roughly 750 miles each way, 1,500 miles round-trip. You're in a minivan rated at 26 MPG highway, but loaded with luggage and four people you're realistically seeing 23 MPG. Gas along the I-95 corridor is averaging about $3.40.
- Gallons needed: 1,500 ÷ 23 = 65.2 gallons
- Total fuel cost: 65.2 × $3.40 = $221.70
- Cost per person (4 people): $221.70 ÷ 4 = ~$55 each
Now run the same trip at 70 mph instead of 60 mph. Aerodynamic drag grows with speed, so highway
economy drops — realistically to around 20 MPG. That makes it 1,500 ÷ 20 = 75 gallons,
or 75 × $3.40 = $255 — about $33 more, just for driving faster.
Use the Gas Trip Cost Calculator to plug in your exact distance and MPG. If you want to know cost per mile rather than total trip cost, the Cost Per Mile Calculator breaks it down further.
Finding your real MPG
The single biggest variable in the formula is MPG, and most people overestimate theirs. Here are the fastest ways to get a reliable number.
- Trip computer average. Most cars since 2010 display an average fuel economy reading — reset it at the start of a highway leg for a highway-specific figure.
- Manual tank-to-tank calculation. Fill up, reset the odometer, drive until low, fill up again. Divide miles driven by gallons pumped. This is the most accurate method.
- EPA fuel economy site. fueleconomy.gov lists real-world user-reported MPG averages by model year, which skew closer to real conditions than the window sticker.
For a heavily loaded road trip, start with your highway EPA rating and subtract 10–15% to account for the load and sustained high speed. That rough adjustment is usually more accurate than using the sticker verbatim. The MPG Calculator can calculate your exact MPG from a fill-up.
How driving conditions shift the cost
| Condition | Effect on MPG |
|---|---|
| Every 5 mph above 60 mph | Roughly −5 to −8% fuel economy |
| Cold start (short trips) | Significant penalty on short drives; minimal on long highway runs |
| Roof box or cargo carrier | Can reduce highway MPG by 5–25% depending on size and shape |
| Underinflated tires (5 psi low) | Roughly −1 to −2% per tire |
| AC running | Roughly −1 to −5% depending on outside temp and system |
| Extra 100 lbs of cargo | Roughly −1 to −2% fuel economy |
These effects stack. A trip with a roof box, loaded cargo, AC on, and 75 mph highway speeds can be 20–30% less efficient than the same car cruising empty at 60 mph.
Easy ways to cut the bill
- Ease off the highway speed. Most cars hit peak fuel efficiency around 55–65 mph; fuel economy falls off noticeably above that. Cruise control also helps by smoothing out subtle accelerations that a human foot causes.
- Check tire pressure before you leave. Underinflated tires quietly cost you MPG and increase wear. Check cold pressure against the door-jamb sticker (not the tire sidewall maximum, which is a different number).
- Lose the roof cargo. A roof box or rack adds significant aerodynamic drag at highway speed — remove it when you don't need it.
- Lighten up and combine stops so you're not hauling dead weight or making cold starts from half-full stops.
- Look ahead for gas prices. Apps like GasBuddy let you plan which exit to stop at — a $0.20/gallon difference on a 60-gallon fill-up saves $12 for a few minutes of planning.
Splitting costs fairly on a group trip
The formula above gives you the total fuel bill. To split it, just divide by the number of people paying — but it's worth deciding up front whether you're splitting equally or by contribution (e.g., the driver might pay nothing, or the person who suggested the trip covers a larger share). Neither approach is wrong; the calculation is the same either way.
If one person is paying for gas and others are Venmoing them back, the most accurate approach is to total up actual receipts rather than using an estimate — prices can vary along a route, and a few stops at expensive rest-stop stations can add up.
Common mistakes
- Using the combined EPA rating for a highway-heavy trip. The EPA combined number blends city and highway. For a long highway drive, use the EPA highway rating (or better, your own measured highway average) — it's usually 3–6 MPG higher than combined.
- Forgetting to double for round trips. The formula gives one-way cost. Double the miles (or the result) if you're driving back. Easy to forget when you're just planning the "fun" direction.
- Assuming gas price stays constant. Gas prices vary by region — a trip through rural stretches or across state lines can mean $0.30+ per gallon swings. Budget a small buffer above your estimated average.
- Ignoring the warm-up penalty for short legs. If your trip involves many short segments (in-and-out of destinations), cold-start fuel penalties add up in a way that a simple miles-divided-by-MPG formula won't capture. For mostly-highway trips this doesn't matter.