How is this calculated
Its quite simple - every time I fill Bitsa with diesel, I make a note on my phone of the indicated mileage, the price per litre, and the quantity used.
Then every couple of weeks I copy this data into a little computer widget I wrote, which is hidden in the depths of this website, which keeps a log of the fuel I've used since the year dot.
It the totals up the fuel, works out what that represents in gallons (original measurements are in litres, to two decimal places), finds the lowest and highest mileages entered, works out the difference, divides the miles covered by the fuel used, and spits the result out.
Although I record what the fuel cost each fill, I don't currently do anything with that figure, its too depressing.
They way I log the fuel longterm is more reliable than just trying to keep a record of one trip, but there are a few downsides. Any fuel loss from faults or maintaince (e.g. fuel hoses spliting, changing the fuel filters) gets recorded just the same as the fuel burnt in the engine. This gives a 'real world' figure, but it is obviously lower than it would be if I was recording the 'offical' consumption over a trip. It also means the result varies a little depending on how much I filled the tank at the last fillup recorded. This causes major swings in the result displayed if the counters have been reset recently, but after 5000 miles, the contents of a full tank make about 0.1 MPG difference to the total displayed.
When did this record start
I've been recording the MPG since mid 2009, but the counter was last reset at the begining of March 2010, when I installed the rebuilt 200di for the second (and hopefully final) time.
Current records suggest fuel consumption is very consistant, at around 25mpg. This is a lot less than ideal, but not entirely unsupprising given the way Bitsa is used, a horrible mix of short trips, a lot of heavy loads over steep mountain passes, and most longer runs being solid fast lane down the motorway, none of which is ideal. If I took to doing long runs in rural Norfolk, rather than thrashing across the Peaks, I'd expect to see at least 30mpg. One major hope is that the motorway consumption will improve dramatically with turbocharging and higher ratio differentials...