Find your real cost per mile and annual fuel spend. Compare two vehicles, or regular vs premium, side by side.
The real cost of fuel isn't just the price at the pump - it's the price divided by your fuel economy. A car with 15 MPG costs the same per mile as a car with 30 MPG if the 30 MPG car uses premium at twice the price.
Follow the minimum octane required by the vehicle manufacturer or tune. The calculator can compare observed costs, but it cannot determine whether regular fuel is permitted or predict an engine's knock control and fuel economy.
A car that gets 25 MPG on premium at $4.29/gal costs 17.2¢/mi. If the same vehicle is approved to use regular and actually averages the same 25 MPG at $3.79/gal, the cost is 15.2¢/mi, a $300 annual difference at 15,000 miles. Use measured MPG for each permitted fuel instead of assuming the economy will stay the same.
EPA labels are standardized estimates for comparing vehicles, not a promise of one driver's result. Weather, trip length, speed, traffic, load, tires, fuel, modifications, and driving style all matter. For budgeting, divide miles driven by gallons added across several complete fill-ups.
US E85 can contain 51% to 83% ethanol, so energy content and fuel economy vary by blend and season. The US Department of Energy notes that an E83 example has about 27% less energy per gallon than gasoline, but a fixed percentage is not reliable for every tank. Enter the actual pump price and your measured MPG on each fuel to find your break-even point.
See the US Department of Energy E85 overview for blend-range and energy-content context.
The two common fuel-economy units measure opposite things. MPG is distance per unit of fuel, so bigger is better. L/100km is fuel burned per fixed distance, so smaller is better. Because they are inverses, you cannot average MPG numbers directly the way you can average L/100km figures. The conversion runs through a single constant:
So 30 US MPG works out to about 7.8 L/100km, and 10 L/100km is about 23.5 MPG. The metric mode of this calculator switches every label and does the math directly in liters and kilometers, so you never have to convert by hand - just enter the numbers the way your dash reports them.
Take a daily driver that averages a real-world 25 MPG and a weekend car that averages 16 MPG, both filled with $4.00 fuel, each driven 800 miles a month. The daily driver costs 4.00 ÷ 25 = $0.16 per mile, or 16.0 cents; the weekend car costs 4.00 ÷ 16 = $0.25 per mile, or 25.0 cents. Monthly that is $128 versus $200, and annually $1,536 versus $2,400 - a $864 difference per year for the same miles, purely from economy. Now add a premium tradeoff: if the weekend car requires 93 octane at $4.50 instead of $4.00 regular, its cost per mile rises to 4.50 ÷ 16 = 28.1 cents, adding roughly $270 a year on top. The point of running both cars through the calculator side by side is to see those gaps in real dollars before they show up on a credit-card statement.
Divide the price per gallon by your real-world MPG. A car getting 25 MPG on $4.00 fuel costs 16 cents per mile. Multiply by monthly mileage for monthly spend.
Use the minimum octane required by the vehicle manufacturer or tune. This tool compares cost; it cannot decide whether a lower-octane fuel is allowed or predict how the engine will respond.
They are inverse measures: MPG is distance per fuel and L/100km is fuel per distance, so a higher MPG means a lower L/100km. The conversion is 235.21 ÷ MPG = L/100km, and the same constant works in reverse. For example, 30 US MPG is about 7.8 L/100km. Switching the calculator to metric handles this for you.
Multiply cost per mile by the miles you drive in a year. Cost per mile is price per gallon divided by your real-world MPG, so annual cost = (price ÷ MPG) × annual miles. At 25 MPG, $4.00 fuel, and 12,000 miles a year, that is (4.00 ÷ 25) × 12,000 = $1,920 per year.
EPA labels provide a standardized comparison, while your result varies with driving, weather, load, traffic, tires, fuel, and vehicle condition. For a personal budget, average several complete fill-ups.
It depends on actual ethanol content, pump prices, and measured economy on each fuel. Because US E85 can contain 51 to 83 percent ethanol, compare price divided by real MPG instead of assuming one fixed discount.
Related Calculators