Fuel economy

Package: UOM_PACKAGE. Registered by createEngine(); for a slimmer engine, register it explicitly (see choosing packages).

Fuel economy says how far a vehicle goes on a given amount of fuel. It is written two ways that measure the same thing from opposite ends, and converting between them is not a plain rescale, so the engine handles the turn for you.

Fuel economy is written two ways that mean the same thing: miles per gallon (mpg), where a bigger number is better, and litres per 100 km (l/100km), where a smaller number is better. One is distance per fuel, the other fuel per distance, so they are opposites of each other, and converting between them means taking one over the other, not just rescaling. That is why it needs its own step, and it is done for you:

40 mpg in l/100km // 5.88 l/100km
6 l/100km in mpg // 39.20 mpg
30 mpg in km/l // 12.75 km/l

Going between two distance-per-fuel forms, mpg and km/l, is an ordinary rescale, since both count the same way up; it is only the mpg-to-l/100km pairing that turns over.

US and imperial gallons

There are two gallons. The US gallon is about 3.79 litres; the imperial gallon, the one British and Irish fuel economy figures are quoted in, is about 4.55 litres, a fifth larger. The engine has both: gallon and gal on their own are the US gallon, and imperial gallon is the larger one.

1 gallon in litres // 3.79 litres
1 imperial gallon in litres // 4.55 litres
1 imperial gallon in gallons // 1.20 gallons

So there are two miles per gallon as well. mpg on its own is miles per US gallon. A UK brochure’s figure is miles per imperial gallon, and it is written with the qualifier, before or after: mpg imperial, imperial mpg, mpg uk, mpg UK or UK mpg. It converts both ways, like mpg:

35 mpg imperial in l/100km // 8.07 l/100km
35 UK mpg in l/100km // 8.07 l/100km
8.07 l/100km in mpg imperial // 35.00 mpg imperial
50 mpg uk in mpg // 41.63 mpg

Because the imperial gallon is larger, the same number of miles per gallon is a thriftier car in the UK than in the US, and the difference is not small: 35 imperial mpg is 29.14 US mpg. It is the same answer as writing the miles over an imperial gallon out in full, which also works:

35 mpg imperial in mpg // 29.14 mpg
50 miles / 1 imperial gallon in l/100km // 5.65 l/100km

The boundary: a bare mpg stays the US gallon, the convention the engine has always used, and the engine does not choose a gallon from your locale. A UK figure typed as plain mpg is therefore read as US miles per gallon, so the car appears to use less fuel than it does (4.70 litres per 100 km rather than 5.65).

50 mpg in l/100km // 4.70 l/100km
50 mpg uk in l/100km // 5.65 l/100km