This gas mileage calculator works out your car’s real fuel economy from the distance you drove and the fuel you used. It shows the result in US MPG, UK MPG, litres per 100 km and km per litre, plus the cost per mile or kilometer.
How to measure your gas mileage
- Fill the tank completely and reset your trip meter (or note the odometer).
- Drive normally until you need fuel again.
- Fill the tank completely and note how many gallons or litres it took.
- Enter the distance on the trip meter and the fuel added.
Fuel economy formulas
- MPG = miles driven ÷ gallons used
- L/100 km = litres used ÷ km driven × 100
- US MPG to L/100 km: 235.215 ÷ MPG
- UK MPG to L/100 km: 282.481 ÷ MPG
Worked example
You drive 350 miles and refill with 11.2 US gallons. Your fuel economy is 350 ÷ 11.2 = 31.25 MPG, which is 7.53 L/100 km or 37.53 UK MPG. At $3.50 a gallon, the tank cost $39.20, or about 11.2 cents a mile.
Ways to improve fuel economy
- Keep tires at the pressure on the driver’s door sticker.
- Accelerate smoothly and keep a steady speed; fuel use rises quickly above about 50 mph.
- Remove roof racks and extra weight when not needed.
- Avoid long idling and keep up with maintenance.
Frequently asked questions
What is good gas mileage?
It depends on the vehicle. Many modern compact cars get over 30 MPG, hybrids often 45–55 MPG, and large SUVs and trucks around 15–22 MPG combined.
Why is UK MPG higher than US MPG?
A UK gallon is 4.546 litres and a US gallon is 3.785 litres, so the same car shows about 20% higher MPG in UK gallons.
Why is my real MPG lower than the official figure?
Official ratings come from standard lab tests. Real driving with traffic, hills, cold weather, air conditioning and higher speeds usually uses more fuel.
Is lower L/100 km better?
Yes. L/100 km measures fuel used, so a lower number means better economy, unlike MPG where higher is better.
How do I estimate the cost of a trip?
Use the fuel cost calculator with your MPG and the trip distance.
Related calculators
Sources
Last reviewed: October 2026.