maths
On a long motorway commute, sitting at 80 when possible, I average anything from 52-57, depending on whether I'm enjoying myself or not. If I'm really bored and sit at 60, I get around 65mpg. Think that's happened twice
When I was REALLY bored and cold, commuting in the damp darkness of the M1 near Nottingham, I began to wonder how you worked out mpg from a pump that only told you about litres. Then, in the cold dark night, my mind took this train of thought:
if 1 gallon is 4.54 litres, then:
mpg = (miles x 4.54) / litres of fuel.
So if Marcus got 195 miles on 19 litres,
195 x 4.54 / 19 = 46 mpg
Personally I can never remember how many litres I put in, but do tend to remember roughly the miles, the price of petrol, and how much it cost to fill up.
If you multiply the distance (miles) by the price of fuel (pence per l), and then divide this by the cost (pounds) times 22 (to convert litres to gallons and pence to pounds), this gives you your mpg.
Honest gov.
So ride 115 miles, fill up at 88p per litre, costing you £8.
(115 x 88) / (8 x 22) = 57 mpg
The moral of this story is NEVER allow yourself to start thinking about maths when you are tired and cold, on a long journey down a dark damp road. Never.
MPG 
