Understanding the MPG Calculator
Miles per gallon (MPG) is the standard measure of a vehicle's fuel economy in the United States: how far it travels on each gallon of fuel. This calculator uses the same fill-up method drivers have used for decades, then extends it to show what that fuel economy means in dollars and gallons for a trip you're planning.
The formula
Fuel economy is simply distance divided by fuel consumed:
MPG = Miles driven ÷ Gallons used
To measure it yourself: reset the trip odometer when you fill the tank, drive normally, then fill up again. The miles shown on the trip odometer are your miles driven; the amount it takes to top off the tank again is your gallons used. Dividing the two gives MPG for that tank.
Cost per mile and trip planning
Once you know MPG, two more numbers fall out directly:
- Cost per mile = price per gallon ÷ MPG. This is what each mile actually costs in fuel, and it's the fairest way to compare running costs between two vehicles with different fuel economy.
- Fuel needed for a trip = trip distance ÷ MPG, and trip fuel cost = fuel needed × price per gallon. Enter a planned trip distance to see roughly how many gallons and dollars it will take.
Why MPG varies
Fuel economy is not a fixed property of a car — it shifts with driving conditions. Highway cruising at moderate speed typically returns better MPG than stop-and-go city traffic. Cold weather, underinflated tires, roof cargo, aggressive acceleration, and heavy air-conditioning use all reduce MPG. Because any single tank reflects one mix of these conditions, averaging MPG over several fill-ups gives a more representative number than one calculation alone.
A worked example
Suppose you drive 300 miles and it takes 10 gallons to refill the tank: MPG = 300 ÷ 10 = 30 MPG. At $3.50 per gallon, cost per mile = 3.50 ÷ 30 ≈ $0.117. For a planned 500-mile trip: fuel needed = 500 ÷ 30 ≈ 16.67 gallons, and estimated fuel cost = 16.67 × 3.50 ≈ $58.33.