MPG vs L/100km: Understanding Fuel Efficiency Conversion

Comparing car fuel economy across borders is headaches-inducing. The US/UK uses MPG (Miles Per Gallon), while the rest of the world uses L/100km (Liters per 100 Kilometers). They measure the same thing, but completely backwards.

Quick Answer (TL;DR)

• MPG measures Distance: "How far can I go on one tank?" (Higher is better).
• L/100km measures Consumption: "How much fuel does it take to go somewhere?" (Lower is better).

Conversion Formula:

235.21 ÷ MPG (US) = L/100km

235.21 ÷ L/100km = MPG (US)

Example: 30 MPG ≈ 7.8 L/100km.

The Two Philosophies

1. Economy (MPG - Imperial)

Origin: North America, UK. Philosophy: "I have a fixed amount of fuel (a gallon). How much work (miles) can I get out of it?"

• Pros: Intuitive for range. "I have 10 gallons, I can go 300 miles."
• Cons: Misleading for savings. The difference between 10 MPG and 20 MPG saves WAY more gas than 30 MPG to 40 MPG (The "MPG Illusion").

2. Consumption (L/100km - Metric)

Origin: Europe, Asia, Canada, Australia. Philosophy: "I have a fixed distance to travel (100 km). How much will it cost me in fuel?"

• Pros: Linear relationship with cost/carbon footprint. Halving the L/100km exactly halves your fuel bill.
• Cons: Slightly harder to calculate range mentally.

The MPG Illusion

Why L/100km is scientifically better for comparing savings.

Scenario A: You replace an old truck (10 MPG) with a new truck (20 MPG).

• Increase: +10 MPG.
• Fuel saved over 100 miles: 10 gal -> 5 gal. Saved 5 gallons.

Scenario B: You replace a sedan (30 MPG) with a hybrid (40 MPG).

• Increase: +10 MPG.
• Fuel saved over 100 miles: 3.33 gal -> 2.5 gal. Saved 0.83 gallons.

Conclusion: Both improved by "10 MPG", but Scenario A saved 6x more fuel! L/100km reveals this instantly (23.5 L/100km drop vs 1.9 L/100km drop).

US Gallons vs UK Gallons (The Trap)

Beware! A UK Gallon is 20% larger than a US Gallon.

• US MPG: Based on 3.785 Liter gallon.
• UK MPG: Based on 4.546 Liter gallon.

Car Spec: A car listed as "50 MPG" in the UK is only getting 41.6 MPG in the US.

• Always check which "gallon" the review is talking about!

Quick Reference Table (US MPG)

Conclusion

• If you want to know Range, think in MPG.
• If you want to know Cost/Pollution, convert to L/100km.
• Use the magic number 235.2 to convert both ways.

Last Updated: February 5, 2026
Reading Time: 6 minutes