Higher MPG means lower L/100km — they run in opposite directions
MPG measures distance per unit of fuel, so bigger is better. L/100km measures fuel per fixed distance, so smaller is better. They describe the same efficiency from opposite ends, which is why the conversion is a division, not a multiplication.
Double the MPG and the L/100km halves. The efficient car (40 MPG) has the shorter bar, because it uses half as much fuel to cover the same 100 km.
The one factor to remember
The constant comes from a US gallon (3.7854 L) and a mile (1.6093 km). For mental math, just divide 235 by the number: 30 MPG is about 235 ÷ 30 = 7.8 L/100km. It works both ways, so 8 L/100km is about 235 ÷ 8 = 29 MPG.
Common values, side by side
| MPG (US) | L/100km | MPG (UK) | km/L |
|---|---|---|---|
| 15 | 15.7 | 18.0 | 6.4 |
| 20 | 11.8 | 24.0 | 8.5 |
| 25 | 9.4 | 30.0 | 10.6 |
| 30 | 7.8 | 36.0 | 12.8 |
| 40 | 5.9 | 48.0 | 17.0 |
| 50 | 4.7 | 60.0 | 21.3 |
The 2024 US new-vehicle average is about 27 MPG, near 8.6 L/100km. In L/100km you look for the smallest number, the reverse of MPG shopping.
Two things that trip people up
- US vs UK gallons. An imperial gallon is about 1.2 US gallons, so UK MPG reads roughly 20 percent higher than US MPG for the identical car. Mixing them overstates efficiency.
- MPGe is not MPG. Electric-car ratings use energy equivalence (1 MPGe = 33.7 kWh), so an 100 MPGe EV does not literally match a 100 MPG gas car at the pump.
Common questions
What is the formula for MPG to L/100km?
L/100km = 235.215 divided by MPG (US). The same formula runs in reverse: MPG = 235.215 divided by L/100km. So 30 MPG is 7.8 L/100km, and 40 MPG is 5.9 L/100km.
Why does a higher MPG give a lower L/100km?
They measure opposite things. MPG counts distance per unit of fuel, so more is better. L/100km counts fuel per fixed distance, so less is better. As one climbs the other falls, which is why you divide rather than multiply.
Is US MPG the same as UK MPG?
No. The imperial gallon is about 20 percent larger, so UK MPG is roughly 1.2 times the US figure for the same car. 30 US MPG is about 36 UK MPG. Always check which gallon a number uses before converting.
