I have a literary mind. Any maths beyond the simple makes very little sense to me, no matter how hard I try to get my head round it.

So, I was somewhat dismayed when I realised, upon taking delivery of my first long-term car last month, that I would, for the first time ever, have to start paying attention to my car’s fuel economy beyond what the optimistic trip computer said.

This seemed simple – even I can do division. But when I came to work out my first interim fuel economy between brims, I realised that it would be a lot more complicated than I'd thought.

While we buy petrol and diesel in the UK in litres, a metric (see: logical) unit, we measure its efficiency in miles per gallon, an imperial (see: irritatingly British) unit.

Perhaps it makes some sense to those who grew up with imperial measurements, but I was born when John Major was prime minister, so the concept of a gallon is about as comprehensible to me as… well, why the actual hell a hundredweight isn’t equivalent to 100 pounds.

Who came up with the imperial system? Were they were some kind of sadist?

Autogas180915Anyhow, what my realisation meant was this: I don’t properly understand how efficient my car is because I don’t know exactly what a gallon is, even if I have a rough idea and know that 12mpg is rubbish and 50mpg is great.

Actually, thanks to the internet, I now do now know what exactly a gallon is: it’s 4.54609 litres. Oh, for…

This meant that in order to work out my car’s MPG, I have to convert the litres I buy into gallons. So last night’s 30.56-litre fill was 6.72226022802 gallons. I had done 324 miles in that time, giving me 48.980746073mpg. Fortunately, my iPhone has a calculator app on it.