How to Calculate Fuel Consumption and MPG
Quick Answer
- Miles per gallon (MPG): MPG = Miles driven ÷ Gallons used
- Litres per 100 km (L/100km): L/100km = (Litres used ÷ km driven) × 100
Example: 40 litres used over 500 km. L/100km = (40 ÷ 500) × 100 = 8 L/100km. Convert to MPG (US): 235.21 ÷ 8 = 29.4 mpg (US).
Understanding your car's fuel consumption helps you budget for fuel costs, compare vehicles before buying and identify when fuel efficiency drops. Use the free Fuel Consumption Converter on CalConvs for instant conversions between MPG, L/100km and km per litre.
How to Calculate Your Car's Real World MPG
- Fill your tank completely. Note the odometer reading.
- Drive normally until you need to refuel.
- Fill the tank again. Note the litres or gallons added.
- Note the new odometer reading.
- MPG (US) = Miles driven ÷ US gallons used
- MPG (UK) = Miles driven ÷ UK gallons used
- L/100km = (Litres used ÷ km driven) × 100
Example: You drove 450 miles and used 15 US gallons. MPG = 450 ÷ 15 = 30 mpg (US).
MPG to L/100km Conversion Table
| MPG (US) | L/100km | Context |
|---|---|---|
| 15 mpg | 15.68 L/100km | Heavy truck or old SUV |
| 20 mpg | 11.76 L/100km | Large SUV or older car |
| 25 mpg | 9.41 L/100km | Average US car or light truck |
| 30 mpg | 7.84 L/100km | Efficient petrol car |
| 35 mpg | 6.72 L/100km | Good fuel economy |
| 40 mpg | 5.88 L/100km | Hybrid or small efficient car |
| 50 mpg | 4.70 L/100km | Hybrid or small diesel |
| 40 mpg (UK) | 7.06 L/100km | Note: UK mpg uses larger gallon |
| 50 mpg (UK) | 5.65 L/100km | Common UK fuel economy figure |
How to Calculate Fuel Cost for a Journey
Fuel cost = (Distance ÷ 100) × L/100km × Price per litre
Example: 600 km journey, car uses 7 L/100km, fuel costs 1.80 per litre. Fuel cost = (600 ÷ 100) × 7 × 1.80 = 6 × 7 × 1.80 = 75.60 (local currency).
Alternative using MPG: Fuel cost = (Miles ÷ MPG) × price per gallon. Example: 300 miles at 30 mpg, fuel at $3.50/gallon = (300 ÷ 30) × 3.50 = $35 USD.
Fuel Economy by Country
- United States: MPG using US gallons. EPA city and highway ratings for all new cars. Average new car approximately 28 mpg.
- United Kingdom: MPG using the larger UK gallon and L/100km. WLTP ratings for new cars since 2018. UK figures naturally higher than US figures for the same car.
- Australia: L/100km standard. Average new car around 7 to 9 L/100km.
- India: Fuel economy in km per litre (kmpl). Average car 12 to 18 kmpl.
- Pakistan: Km per litre. Common vehicles like the Suzuki Alto achieve 18 to 22 kmpl in city driving.
- Europe: L/100km under WLTP testing. Average new European car approximately 6.5 to 7.5 L/100km.
Frequently Asked Questions
How do I convert km per litre to L/100km?
L/100km = 100 ÷ km per litre. Example: 15 kmpl = 100 ÷ 15 = 6.67 L/100km.
What is a good fuel consumption figure?
For petrol cars: below 7 L/100km (over 40 mpg US) is excellent. 7 to 9 L/100km is good. Above 12 L/100km is poor. For diesel cars these figures are typically 15 to 20% lower. Hybrids achieve 4 to 5 L/100km or better.
Why are UK and US MPG figures different for the same car?
Because the UK gallon (4.546 litres) is larger than the US gallon (3.785 litres). A car rated at 40 mpg on the UK scale is only 33.3 mpg on the US scale. Always check which country a fuel economy rating comes from.
How do I calculate fuel cost for a road trip?
Find your car's L/100km figure, look up the current fuel price per litre and apply the formula above. Use the Fuel Consumption Converter to convert between MPG and L/100km if needed.
Related Tools
- Fuel Consumption Converter: MPG (US), MPG (UK), L/100km and km per litre
- Length Converter: km to miles for journey distance calculations
- All Unit Converters: 74 converters on CalConvs