The MPG Obfuscation

Petrol and diesel prices in the UK have been metricised to pence per litre for as long as I can remember (I would guess almost 20 years). Cars in the UK still use MPG in manuals and electronic displays. And the common language is certainly still in terms of MPG. Car manuals also quote efficiency figures in litres per 100km or similar - but that is still obfuscated by the fact that we use miles for judging distances.

I want to easily get to a pence per mile figure.

So here are the figures...

• 1 imperial gallon = 4.546 litres
• 1 mile = 1.609km

Using dimensional analysis you can see that to get to pence per mile you need to multiply your pence-per-litre by litres-per-gallon and divide by miles-per-gallon. Here's a table showing pence per mile for a few typical values (at the moment).

	20mpg	30mpg	40mpg	50mpg	60mpg
100p	23	15	11	9	8
105p	24	16	12	10	8
110p	25	17	13	10	8
115p	26	17	13	10	9
120p	27	18	14	11	9
125p	28	19	14	11	9
130p	30	20	15	12	10
135p	31	20	15	12	10
140p	32	21	16	13	11

There's a lot to be said for reviewing the 40p per mile tax-exempt approved mileage allowance (for business usage). This was set many years ago before the recent significant rises in fuel prices.