Categories🚗 Auto & TransportFuel Economy Calculator

Fuel Economy Calculator

Convert and calculate fuel efficiency in US MPG, Imperial MPG, and L/100 km.

Calculated Fuel Efficiency
US MPG
29.167
Imperial MPG
35.028
L/100 km
8.065
Guide & Educational Reference
How to Use the Fuel Economy Calculator
  1. 1Enter the distance driven (in miles or kilometres).
  2. 2Select your distance unit dropdown (Miles or km).
  3. 3Enter total fuel consumed and select fuel unit (US Gal, UK Gal, or Litres).
  4. 4View your calculated fuel efficiency simultaneously converted into US MPG, Imperial MPG, and L/100 km.
Understanding Fuel Economy Metrics

Fuel economy measures how efficiently a motor vehicle converts fuel into distance travelled. Different regions use different standards: the US uses Miles per US Gallon (US MPG), the UK uses Miles per Imperial Gallon (Imp MPG), and metric countries use Litres per 100 Kilometres (L/100 km).

Formulas Implemented by THE CALC
Unit Conversions

Internal conversions implemented in autoEngine.js:

1 mi = 1.60934 km | 1 US Gal = 3.78541 L | 1 UK Gal = 4.54609 L = 1.20095 US Gal
US MPG

Miles travelled per US Gallon consumed.

US MPG = Miles / US Gallons
Imperial MPG

Miles travelled per UK Gallon consumed.

Imperial MPG = Miles / UK Gallons
L/100 km (Metric)

Litres consumed per 100 Kilometres travelled.

L/100 km = (Litres / Kilometres) × 100
Practical Real-World Example
350 Miles Driven on 12 US Gallons
US MPG = 350 / 12 = 29.17 US MPG Imp Gal = 12 / 1.20095 = 9.99 Im Gal → Imp MPG = 350 / 9.99 = 35.03 Imp MPG 563.27 km / 45.42 L → L/100 km = (45.4249 / 563.269) × 100 = 8.06 L/100 km
→ US MPG: 29.17 | Imperial MPG: 35.03 | L/100 km: 8.06
400 miles on 12.5 US gallons
US MPG = 400 / 12.5 = 32; L/100km = (12.5 × 3.78541 / (400 × 1.60934)) × 100
→ 32 US MPG (≈ 7.35 L/100 km)
600 km on 40 litres
L/100km = (40 / 600) × 100 ≈ 6.67; US MPG = (600 × 0.621371) / (40 / 3.78541)
→ ≈ 6.67 L/100 km (≈ 35.28 US MPG)
Important Considerations & Directionality
  • Inverse relationship: Higher numbers mean better efficiency for MPG (more miles per gallon), but lower numbers mean better efficiency for L/100 km (fewer litres needed per 100 km).
  • US vs UK Gallon difference: Because a UK Imperial gallon is ~20% larger than a US gallon, the same vehicle will show a ~20% higher numerical figure in Imperial MPG than in US MPG.
  • Full-tank measurement technique: To measure fuel economy accurately manually, fill your gas tank completely, reset trip odometer, drive normally, refill to full, and divide trip mileage by gallons filled.
Frequently Asked Questions

What is a good L/100 km rating?

For gasoline passenger vehicles, under 6.0 L/100 km is considered excellent (high efficiency/hybrid), 6.0–8.0 L/100 km is good, and above 10.0 L/100 km indicates high fuel consumption.

Why does Imperial MPG differ from US MPG?

An Imperial gallon is defined as 4.54609 litres, while a US gallon is 3.78541 litres. Driving the same distance on 1 Imperial gallon uses more fuel than 1 US gallon, resulting in a higher MPG ratio.

What outputs does it give?

US MPG, imperial (UK) MPG, and litres per 100 km, all computed from the same distance and fuel amount after converting units internally.

Why are US and imperial MPG different?

An imperial gallon (≈ 4.546 L) is larger than a US gallon (≈ 3.785 L), so the same fuel volume yields a higher imperial MPG figure.

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