How it works
In US mode, efficiency is miles per US gallon, so a bigger number means less fuel for the same distance. In metric mode it's liters per 100 kilometers, so a smaller number means less fuel. Switching units converts the distance, fuel price and efficiency together, with small rounding differences.
The trip estimate and the annual estimate use the same fuel economy and price but separate distances. Monthly spending is the annual estimate divided by twelve. That lets you budget one particular trip without assuming you take it every day. Money stays in 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 burns about 10 US gallons. At an illustrative $3.50 a gallon, that's $35 in fuel. Drive 12,000 miles a year on the same numbers and fuel runs $1,400 a year, or about $117 a month. These are arithmetic examples, not today's pump prices.
How do I make the estimate match my driving?
Official fuel-economy ratings are a good starting point for comparing cars, but real driving differs from the test cycle. Short trips, cold starts, traffic, speed, tire condition, cargo and hills all change how much fuel you use. If you track trip distance and fuel added across several fill-ups, that longer record is usually a better input than one good trip.
Don't mix imperial mpg with US mpg. An imperial gallon is bigger, so the same car shows a different mpg figure. This tool uses US gallons and liters only, so convert imperial numbers before entering them. The kilometers setting changes units; it doesn't look up Canadian or UK fuel prices.
How do I compare efficiency in dollars?
The link between mpg and fuel use isn't a straight line. Over the same distance, going from 20 to 25 mpg saves more gallons than going from 40 to 45. Enter both scenarios and save one for comparison to see the real spending difference. Fuel savings are only part of choosing a car, and an expensive upgrade may take many years to pay for itself that way.
The sensitivity table moves the pump price 20% above and below your input while distance and efficiency stay fixed. It's a what-if comparison, not a forecast. For a full budget, add maintenance, depreciation, insurance and financing in the ownership-cost calculator. Electric charging and plug-in hybrids need their own energy models and are outside this tool.
Questions worth asking
Are fuel prices updated automatically?
No. Enter a pump price or a planning assumption. The default is only an illustration and isn't 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 stay in USD, so convert to a USD-equivalent price if you need to.
Why did my numbers change when I switched units?
Distance, efficiency and price are converted so the scenario stays roughly the same. Rounding can cause a small difference in the cost you see.
Can I enter zero miles?
Yes. A zero-distance trip costs zero. Efficiency does have to stay above zero so the fuel consumption means something.
Does this estimate electric charging costs?
No. It models liquid fuel only. Electric cars need kWh use, charging losses and electricity prices instead of mpg.
This is general information, not personalized financial advice. Read the limitations.
