Step 01
Keep the gallon standard explicit.
UK MPG uses the imperial gallon of about 4.546 litres. US MPG uses the US gallon of about 3.785 litres. The same distance figure therefore converts to different litres per 100 km.
Always confirm which standard a specification uses before comparing vehicles.
Step 02
Understand the conversions.
The converter shows all four outputs from one input, making it easy to spot an inconsistent specification.
- UK MPG → L/100 km: 282.481 ÷ MPG.
- US MPG → L/100 km: 235.215 ÷ MPG.
- km/L → L/100 km: 100 ÷ km/L.
- L/100 km → km/L: 100 ÷ L/100 km.
Step 03
Worked example: 45 UK MPG.
45 UK MPG is about 6.28 L/100 km, 15.93 km/L and 37.5 US MPG. The US figure is lower because the US gallon is smaller.
If a vehicle is quoted at 45 MPG without saying which standard, the litres per 100 km could be interpreted differently.
Step 04
Compare vehicles on one standard.
Use the worksheet to record each vehicle's quoted unit and converted values on the same standard. Real-world economy still depends on driving conditions, load, weather and style.
A converted figure is a comparison aid, not a guarantee of actual fuel use.
Free reusable resource
Keep a practical worksheet.
Download a blank, editable file and keep the plan with your own records. No email address is required.
Fuel economy worksheet
Record the value, source unit and all converted results together with the vehicle and route context.