BYTETOOLS

Fuel Economy Converter

Convert MPG US, MPG imperial, L/100 km, km/L and miles per litre in any direction, then cost a journey and see the real saving between two figures.

30.00
MPG (US)
36.03
MPG (imp)
7.84
L/100 km
12.75
km/L
7.93
mi/L

Every unit, and the constants behind them

  • Miles per US gallon (MPG US)30.0000
  • Miles per imperial gallon (MPG UK)36.0285
  • Litres per 100 km (L/100 km)7.8405
  • Kilometres per litre (km/L)12.7543
  • Miles per litre (mi/L)7.9252
  • MPG (US) × L/100 km = 235.214583 — that is 3.785411784 L per US gallon ÷ 1.609344 km per mile × 100
  • MPG (imperial) × L/100 km = 282.480936 — the same sum with the 4.54609 L imperial gallon
  • km/L × L/100 km = 100
  • Both gallon definitions and the mile are exact by definition, so these constants are exact too.

What it costs to drive

  • At 30 MPG (US): 1,514.2 L (400.0 US gal) costing $1,400.00
  • At 40 MPG (US): 1,135.6 L (300.0 US gal) costing $1,050.00
  • Difference: 378.5 L and $350.00 saved by the second figure

Why 15 → 20 MPG beats 30 → 40 MPG

MPG is fuel per distance inverted, so equal MPG steps are not equal fuel savings. Over 12,000 miles:

  • 15 → 20 MPG (US)saves 200.0 US gal (757 L)
  • 30 → 40 MPG (US)saves 100.0 US gal (379 L)

A 5 MPG gain at the bottom of the range saves twice the fuel of the same gain at the top. L/100 km does not have this problem, which is why most of the world quotes consumption rather than economy.

What is the Fuel Economy Converter?

The ByteTools Fuel Economy Converter moves between all five common measures using the exact constants: 235.

  • MPG (US), MPG (imperial), L/100 km, km/L and miles per litre, all directions
  • Exact constants 235.214583 and 282.480936, with the derivation spelled out
  • Journey cost in any currency, priced per litre or per gallon
  • Side-by-side comparison showing the fuel and money a change is worth
  • Worked demonstration of why 15 → 20 MPG beats 30 → 40 MPG
  • Distance in miles or kilometres, and results in litres and US gallons

How to use the Fuel Economy Converter

  1. 1

    Type your fuel economy figure and choose the unit it is measured in.

  2. 2

    Leave the comparison dropdown on to enter a second figure, or switch it off to just convert.

  3. 3

    Read all five units in the summary tiles, with the exact constants shown below them.

  4. 4

    Enter your annual distance, the fuel price and how it is priced to cost the driving.

  5. 5

    Compare the two figures side by side and see the fuel and money the difference is worth.

About the Fuel Economy Converter

The ByteTools Fuel Economy Converter moves between all five common measures using the exact constants: 235.214583 for US MPG against L/100 km, and 282.480936 for the imperial gallon. Both gallon definitions and the international mile are exact by definition, so these are not approximations — they are the true conversion factors carried to full precision.

Beyond conversion it costs the driving. Enter an annual distance and a fuel price — per litre, per US gallon or per imperial gallon — and you get the fuel used and the money spent. Enter a second economy figure and it shows the litres and the cash the difference is worth over that distance.

It also demonstrates the MPG illusion with your own numbers: going from 15 to 20 MPG saves twice as much fuel as going from 30 to 40, because MPG is an inverted measure. Everything runs locally in your browser and nothing is uploaded.

Frequently asked questions

How do you convert MPG to L/100 km?

Divide 235.214583 by the MPG figure. So 30 MPG (US) is 235.214583 ÷ 30 = 7.84 L/100 km. The constant comes from 3.785411784 litres per US gallon divided by 1.609344 km per mile, times 100 — all three of those numbers are exact by definition.

What is the difference between US and imperial MPG?

The imperial gallon is 4.54609 litres against the US gallon's 3.785411784, about 20% larger, so the same car reads roughly 20% more MPG in the UK than in the US. A car doing 30 MPG (US) does 36 MPG (imperial) — the conversion constant is 282.480936 rather than 235.214583.

Why does 15 to 20 MPG save more fuel than 30 to 40 MPG?

Because MPG is distance per fuel, not fuel per distance. Over 12,000 miles, 15 MPG burns 800 gallons and 20 MPG burns 600, saving 200. But 30 MPG burns 400 and 40 MPG burns 300, saving only 100. This is the MPG illusion, and it is why L/100 km is the more honest measure.

Is a higher or lower L/100 km better?

Lower is better, because it is a consumption figure — the litres needed to cover 100 km. That is the opposite of MPG, where higher is better. A car at 5 L/100 km is efficient; one at 12 L/100 km is thirsty.

How do I calculate my fuel cost per year?

Divide your annual distance by your economy to get the fuel used, then multiply by the price. This tool does it in one step: enter the distance, the price and how it is priced, and it reports litres, US gallons and the total cost in whatever currency symbol you type in.

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