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.
Premium (91-93 octane) is required for many high-performance engines - using regular can trigger knock retard, which reduces both power and efficiency. However, if your car's ECU can safely adapt to 87 octane, the savings may not justify the switch.
A car that gets 25 MPG on premium at $4.29/gal costs 17.2¢/mi. The same car on regular at $3.79/gal is 15.2¢/mi. At 15,000 miles per year, that's $300/year. If knock retard costs you 5 MPG (dropping to 20 MPG), the "savings" evaporate - you're now spending $189 more per year using regular.
EPA estimates typically overstate real-world fuel economy by 10-20% for performance cars, especially in urban driving or spirited use. If your car is rated 22/32 MPG, budget for 18-20 MPG in mixed driving. Track or autocross days can push consumption to 10-14 MPG regardless of the EPA rating.
E85 contains about 30% less energy than gasoline per gallon. At the same pump price, you'd need E85 priced at least 30% below regular for it to break even on fuel cost alone - not counting the power gains from tuning for E85's higher octane rating.
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.
If the engine requires premium, running regular triggers knock retard that reduces power and efficiency - often erasing the pump savings. If the engine is designed for regular, premium typically buys nothing measurable.
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 figures are lab estimates and typically overstate real-world economy by 10 to 20 percent for performance cars, more in city driving or spirited use. Cost-per-mile math is only as good as the MPG you feed it, so use the figure you actually average from fill-up to fill-up rather than the window sticker.
E85 holds about 30 percent less energy per gallon than gasoline, so your MPG drops by roughly that much. For E85 to break even on fuel cost alone it needs to be priced at least 30 percent below the gasoline you would otherwise burn - and that ignores any power gains from tuning for its higher octane.
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