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L/100 km to MPG and vice versa

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Phil74Camper Head in the Clouds

2,703 posts · joined Aug 2002 ·

#761818
There seems to be quite a lot of confusion about how to convert fuel economy between the different units, especially as there are two different-sized gallons to be aware of.

For example, the latest VWMA issue #25 reports on the new VW Compact Coupe (page 97), saying: "Combined fuel consumption is claimed at 4.2 litres per 100 km, or in the world of 'yesteryear' speak, 45 miles to the gallon.'

Sorry, wrong.

To convert MPG (Imperial, or British gallon) into L/100 km, you take the constant of 282 and divide the figure into it. It works both ways:

L per 100 km = 282 / MPG
MPG = 282 / L per 100 km

In this case, 4.2 L/100 is (282/4.2) = 67.1 mpg.

And back again: 282/67.1 = 4.2.

In case you are wondering about this piece of mathematical black magic - where does 282 come from? Well there are 1.61 km in 1 mile, and there are 4.546 litres in 1 imperial gallon. 1.61 divided by 100, and divided by 4.546, and take the inverse - 282.36.

US gallons are a bit smaller than Imperial gallons, at only 3.785 litres. The constant you use for US gallons, worked out the same way, is 235.

Even using US gallon conversion, 4.2 L/100 km works out at 55.9 US MPG. So I dunno where '45 mpg' came from...

Bizarre Moderator The artist formerly known as blue74l

12,789 posts · joined Aug 2002 ·

#761835
should be Km/L
Futue te ipsum!!!

Phil74Camper Head in the Clouds

2,703 posts · joined Aug 2002 ·

#761946
Yeah I wondered about that. Miles per gallon - OK, change to metric - litres per kilometre.

But that isn't a measure of fuel consumption, it's a measure of economy. What's the difference?

Well, with mpg or km/L, when the number goes UP - say from 25 to 40 mpg - you are using LESS fuel. It's your 'economy' that is getting bigger. When the number goes DOWN, you are using MORE fuel. That's not particularly logical.

However with L/100 km, it's the other way around. When the numbers go Up - say from 7 to 10 L/100 km, you are using MORE fuel. When the numbers come DOWN, you are using LESS fuel. It's a true measuement of 'consumption' - it tells you how much fuel you consume per a given distance.

Aircraft and boats have always done the same, except in litres or gallons per hour instead.

1303Steve Scirocco Rare

8,650 posts · joined Aug 2002 ·

#761950
Hi

I find L/km easy to work out my fuel consumption, if I've travelled 320 ks between fills and I put 32 litres in I'm getting 10 litres/100.

Steve