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Next time, fill the tank and zero the milecounter. Drive, then when you fill it up again make a note of the number of litres it takes to fill up and the number of miles done - then you can work out the MPG.
This is what Fuelly does. Text mileage, litres and price each time you fill up (to first pump click for consistency) and it will text you back previous mileage, cost per litre, overall mpg etc etc. It also tracks it online.... my current car: http://www.fuelly.com/car/bmw/430d/2015/soul7963/550815
My tank holds 50l I've covered just over 100 miles since my last top up and I'm 1/2 way through it if the petrol gauge is to be trusted. That means I'm doing 18-19mpg and mileage wise I'm about 80% motorway/country lanes and 20% non rush hour city driving.
There must be something pretty wrong with my car I reckon, odd as it appears to run fine and performs better than I was expecting. One thing I noticed is that somebody replaced the stock exhaust with a 1" pea shooter backbox, could this be messing with my fuel consumption?
I'm gonna replace the air filter, plug and backbox hoping that'll claw back some miles to my gallon. I should be getting 29-33mpg according the manual.