Fuel Economy Calculator
MPG and L/100km from trip distance and fuel used, plus trip cost at your fuel price.
What it does
Fuel economy answers one question: how far does each unit of fuel carry you. The United States states it as miles per gallon, where higher is better. Most of the world states it as liters per 100 km, where lower is better. Both describe the same burn from opposite ends.
The tool above takes a trip distance and the fuel used, in either unit pair, and returns both ratings at once. Add the pump price to price the same trip. Keep the price unit matched to the fuel unit you picked.
The honest numbers come from full tank to full tank. Fill up, reset the trip meter, drive normally, fill again at the same pump, and enter those two readings. Single dashboard snapshots swing with hills, wind, and idling.
How to use
- Enter the trip distance and pick miles or kilometers.
- Enter the fuel used and pick gallons or liters.
- Enter the price per that same fuel unit, or zero to skip cost.
- Compare fill ups over a month rather than judging one tank.
Formula
- miles / gallons
- US miles per US gallon.
- 235.214583
- Fixed bridge between the two ratings.
- price matching
- Price unit must match the fuel volume unit.
The 235.214583 constant folds the mile, gallon, liter, and 100 km conversions into one step. Cost always returns to fuel volume times price, so matching the price unit to the fuel unit keeps the money right.
Worked example
Example: 300 miles on 10 gallons at $3.50
A highway trip of 300 miles burns 10 gallons. Fuel costs $3.50 per gallon.
- MPG: 300 / 10 = 30 mpg.
- Metric rating: 235.214583 / 30 = 7.84 L/100km.
- Trip cost: 10 x $3.50 = $35.00.
- Per mile: 35.00 / 300 = about $0.117 per mile.
Reference table
| US MPG | L/100km | Typical of |
|---|---|---|
| 20 mpg | 11.76 | Large SUV or truck |
| 25 mpg | 9.41 | Midsize SUV |
| 30 mpg | 7.84 | Midsize sedan |
| 40 mpg | 5.88 | Hybrid sedan |
| 50 mpg | 4.70 | Efficient hybrid |
Limits
- US gallons assumed. Imperial gallons read higher.
- One tank varies with wind, hills, and idling.
- Hybrids and EVs break single fuel math.
- Pump and odometer error add a few percent.
Common mistakes
- Judging one tank with unusual wind, hills, or idling instead of a month of fill ups.
- Mixing imperial gallons (4.546 liters) with US gallons (3.785 liters). This tool uses US gallons.
- Pricing per gallon while entering liters, which miscounts cost by nearly four times.
- Trusting the dashboard alone. Pumps and odometers disagree with trip computers by a few percent.
- Comparing winter and summer tanks directly. Cold starts and winter fuel cost real economy.
Try it yourself
Quick checks. Answers sit tucked under each question.
400 km on 32 L. Ratings?
32 / 4 = 8.00 L/100km. Then 235.21 / 8 = 29.40 mpg.
That trip at $1.60 per liter?
32 x $1.60 = $51.20.
Questions
Is higher MPG or lower L/100km better?
Higher MPG is better and lower L/100km is better. They invert: 30 mpg equals 7.84 L/100km.
US or imperial gallons?
This tool uses US gallons of 3.78541 liters. Imperial gallons are larger, so the same car shows a higher imperial MPG number.
What is the 235.21 number?
A fixed converter between the two ratings. Divide it by MPG to get L/100km, or by L/100km to get MPG.
How do I measure without a trip computer?
Fill the tank, reset the odometer, drive, then refill. Divide trip miles by refill gallons for that tank.
Why did my economy drop in winter?
Cold engines run rich longer, tire pressure falls, and winter fuel blends hold less energy. A drop near 10% is normal.