Re: units
litres per 100 km
All cars these days are sold with fuel consumption statistics in litres per 100 km and miles per gallon. Most newish cars will display their fuel consumption in either unit.
Unfortunately, we measure distance in miles but we buy fuel in litres, so calculating the cost of a journey requires knowledge of the conversion factor* and a little bit of unnecessary mental arithmetic. What we really need is miles per litre.
*For some reason, probably laziness, I never know the litres/gallons conversion, but I can always remember gallons (of water) to pounds, pounds to kilos (approx) and, of course, kilos (of water) to litres. This isn't a good calculation to be doing in traffic, and I suspect my results are too approximate to be useful anyway.