GarageCalc What this calculator does
The math is simple and shown openly: in US units, fuel needed = distance / MPG, and cost = fuel x price per gallon. In metric, fuel needed = distance x (L/100km) / 100, and cost = liters x price per liter. The per-person figure divides the total evenly, which is the standard way to split gas money on a shared road trip.
Note that fuel is only part of the true cost of driving. Tires, oil and depreciation roughly double the per-mile cost of an average car. For splitting a road trip with friends, fuel-only is the usual convention; for comparing driving against flying, use the full per-mile cost.
Getting an honest MPG number
The biggest error source is fuel economy. The EPA sticker is measured at moderate speeds; real interstate driving at 75 mph, with cargo or a roof box, can be 10 to 25 percent worse. Best practice: reset the trip computer on a similar drive beforehand and use that number. Wondering how your tires affect the reading? A taller tire makes the odometer under-count distance, which also skews computed MPG: check yours with the tire size comparison calculator.
Frequently asked questions
How is trip fuel cost calculated?
Fuel needed = distance divided by fuel economy (or distance x consumption per 100 km in metric). Cost = fuel needed x price per gallon or liter. This calculator does both unit systems and splits the total between passengers.
Should I use city or highway MPG?
For road trips, use your highway figure or, better, your real observed average from the trip computer. EPA combined ratings tend to overestimate real highway economy at higher speeds, so expect 5 to 10 percent worse than the sticker on a fast interstate run.
How much does speed affect fuel cost?
Aerodynamic drag grows with the square of speed. Above about 60 mph, each additional 5 mph typically costs 5 to 8 percent in fuel economy, so driving 75 instead of 65 can add roughly 15 percent to the fuel bill on the same route.
Does cargo or a roof box change the math?
Yes, noticeably. A loaded roof box can cost 10 to 25 percent at highway speed because of drag; heavy cargo matters mostly in stop-and-go driving. If the trip is loaded, lower your expected MPG before calculating.