How to use the fuel grade break-even calculator
- Choose miles and gallons or kilometers and liters.
- Enter the price of the cheaper fuel and the price of the dearer one.
- Enter your economy on the cheaper fuel, and optionally the economy you measured on the dearer one and the distance you drive in a year.
- Read the break-even economy the dearer fuel must reach and, if you measured it, the change in your yearly cost.
Formula
price ratio = dearer price / cheaper price
break-even miles per gallon = miles per gallon on the cheaper fuel x price ratio
break-even liters per 100 km = liters per 100 km on the cheaper fuel / price ratio
cost per mile = price per gallon / miles per gallon
Distance per unit of fuel must rise by the same percentage as the price. A 17.65% dearer fuel needs 17.65% more distance from each gallon to cost the same per mile.
Regular at $3.40 and the dearer fuel at $4.00
The price ratio is 4.00 / 3.40 = 1.1765, so the dearer fuel is 17.65% more expensive. A car that gets 25 miles per gallon on the cheaper fuel must reach 25 x 1.1765 = 29.41 miles per gallon on the dearer one to break even.
The cheaper fuel costs $0.1360 a mile. If you measure 26 miles per gallon on the dearer one, it costs $0.1538 a mile, which is $0.0178 more, and over 12,000 miles that is $214.15 more a year. In metric units, a car using 8.4 liters per 100 km on fuel at $1.65 a liter must use 7.49 liters per 100 km on fuel at $1.85 a liter.
How to measure your own economy
One tank can mislead, because driving, weather and how full you fill the tank all change the result. Track several full tanks of each fuel, and use the totals. The MPG Calculator handles several fill-ups, and the Fuel Economy Unit Converter switches between miles per gallon and liters per 100 kilometers.
For a single trip rather than a yearly comparison, the Fuel Cost Calculator and the Road Trip Fuel Stops Calculator use the economy you found.
This is a cost comparison only
Some engines are built for a particular fuel and the maker states what to use. This calculator does not judge that. It answers a single question, which is how much better the economy must be for a dearer fuel to cost the same per mile, and the answer applies only if the engine is allowed to use both.
Assumptions and limits
- Prices stay the same over the year, and the car can use either fuel.
- The economy you enter is measured over enough driving to be reliable.
- The yearly figure multiplies the difference in cost per distance by the distance you enter.
- Prices on this page are examples. Enter the prices you see.
Frequently asked questions
Why does the answer depend only on the price ratio?
Cost per mile is price divided by miles per gallon. Setting two such costs equal means the miles per gallon must change by the same factor as the price, whatever the starting economy.
How do I enter economy in liters per 100 kilometers?
Choose the metric units. Then lower numbers mean better economy, and the break-even figure is the consumption the dearer fuel must stay under.
What if I have not measured the dearer fuel yet?
Leave that box empty. The tool then shows only the target to beat, and you can run a few tanks and come back to fill in the result.
Does the tool consider the price of oil changes?
No. It compares two prices you give it at one moment. If prices move often, repeat the comparison with the prices you see now.