How it works
In US mode, efficiency is miles per US gallon, so a larger number means less fuel for a fixed distance. In metric mode, efficiency is liters per 100 kilometers, so a smaller number means less fuel. The units switch converts the entered distance, fuel price, and efficiency together, with small rounding differences.
The trip estimate and annual estimate use the same fuel economy and price but separate distances. Monthly spending is the annual estimate divided by twelve. This makes it possible to budget a specific journey without accidentally assuming you take that same trip every day. Monetary figures remain USD in either unit system.
The formula behind the estimate
Every result starts with a defined model and visible assumptions.
US gallons used = miles ÷ miles per US gallon Liters used = kilometers × liters per 100 km ÷ 100 Fuel cost = fuel used × price per matching volume unit 1 US gallon = 3.785411784 liters; 1 mile = 1.609344 kilometers
A worked example
A 300-mile trip in a car that averages 30 US mpg consumes about 10 US gallons. At an illustrative $3.50 per gallon, fuel costs $35. For 12,000 annual miles at those same inputs, fuel is $1,400 per year, or approximately $117 per month. These are arithmetic examples, not today’s pump prices.
Make the estimate match your driving
Official fuel-economy ratings are useful comparison inputs, but your driving pattern may differ from the test cycle. Short trips, cold starts, traffic, speed, tire condition, cargo, and terrain can affect actual consumption. If a car records both trip distance and fuel added across several fill-ups, that longer observation can provide a more relevant estimate than one favorable journey.
Do not mix imperial mpg with US mpg. An imperial gallon is larger, so the same physical fuel economy produces a different mpg figure. This tool offers US gallons and liters only. If your source uses imperial units, convert it before entering a US mpg figure. A kilometers setting changes measurement units; it does not retrieve Canadian or UK fuel prices.
Compare efficiency in dollars
The relationship between mpg and fuel use is not linear. For the same distance, improving from 20 to 25 mpg saves more gallons than improving from 40 to 45 mpg. Enter both scenarios and save one for comparison to see the actual spending difference. That difference is only one part of choosing a car; an expensive upgrade may take many years to recover through fuel savings.
The sensitivity table varies pump price by 20% above and below your input while holding distance and efficiency fixed. It is a what-if comparison, not a forecast. For a complete budget, add maintenance, depreciation, insurance, and financing in the ownership-cost calculator. Electric charging and plug-in hybrid operating splits need separate energy models and are outside this tool’s scope.
Questions worth asking
Are fuel prices updated automatically?
No. Enter a pump price or a planning assumption. The default is illustrative and is not described as a current national or local average.
Does metric mode change the currency?
No. It changes distance, consumption, and volume units. Prices and results remain USD, so use a USD-equivalent price if necessary.
Why did numbers change when I switched units?
Distance, efficiency, and price are converted to preserve approximately the same physical scenario. Rounding can produce a small difference in the displayed cost.
Can I enter zero miles?
Yes. A zero-distance trip costs zero in this model. Efficiency must stay above zero to keep consumption meaningful.
Does this estimate electric charging costs?
No. It models liquid fuel only. Electric vehicles require kWh consumption, charging losses, and electricity pricing instead of mpg.
This is general information, not personalized financial advice. Read the limitations.
