Fuel Trip Budgeting: Distance, Efficiency, and Price

The basic math

Fuel cost equals distance divided by efficiency, times the price per gallon. A 300-mile trip in a car getting 30 mpg uses 10 gallons. At $3.50 per gallon, that is $35 — about 12 cents per mile. The calculator does this for any distance, efficiency, and price.

Efficiency varies with conditions

Real-world mpg is lower than the window sticker: highway speeds above 60 mph, headwinds, hills, winter fuel blends, and cargo all reduce efficiency. A loaded SUV towing a trailer can drop below half its rated mpg. Use a conservative number from your own average, not the sticker, for a realistic estimate.

Electric vehicles change the units

For EVs, the equivalent is miles per kWh times the charging rate. A 300-mile trip in a car doing 3.5 mi/kWh uses about 86 kWh; at $0.15 per kWh that is about $13 — often cheaper than gasoline, though public fast charging typically costs more than home rates.

Round trips and detours

Budget the round trip, not the one-way distance, and add margin for detours, wrong turns, and idling in traffic. Adding 20 percent to the fuel estimate covers the difference between the map distance and the distance you actually drive.

Compare before you drive

The calculator makes it easy to compare two vehicles or two routes: same distance, different efficiency or price. Use it to decide which car to take, whether a detour for cheaper gas is worth it, and what to set aside for the trip's fuel.

The per-mile lens

Expressing fuel as cost per mile makes comparisons easy: $3.50 gas at 30 mpg is 11.7 cents per mile; at 20 mpg it is 17.5 cents. Over 12,000 miles a year, the difference is about $700. The same lens works for an EV: 3.5 mi/kWh at $0.15/kWh is about 4.3 cents per mile.

Plan with a buffer

Set the trip fuel budget 15-20 percent above the estimate. Traffic, detours, temperature, and load all reduce efficiency, and a one-way estimate doubled for the return trip is the minimum. The buffer turns a budget from optimistic to usable.

Sources and further reading

• U.S. Department of Energy — fueleconomy.gov
• Wikipedia — Fuel economy in automobiles