first of all awesome mileage, i drove the new city (manual) for a day in city and with my usual driving style got 12.1, get 14 with similar driving on Nissan sunny diesel.
anyway the speedo thing you mentioned, i tested this a lot in numerous cars, the speedo from factory is usually fast, i.e. telling more speed, this error ranged from 9 percent in older civic exi, to 5 percent in the current vti. how i tested it was with gps over at least a couple of km's long constant drive and secondly with time noted between two fixed distances again by gps with accuracy around 3 meter.
thirdly i have a question from all, the instantaous average that the meter shows is while the car is moving, basically calculated by the speed and the injector duty cycle, when the car is stationary at a traffic light their is no average, the overall average is calculated by the instantanous average or with the accumulative injector ON time by distance covered?
lastly what i have noticed is that there is a lot of difference between filling stations, with the needle at the same place i.e. EMPTY(E) i have seen a difference of around 4 liters over 43 liters of total fuel, now this is a lot, hence what i do is i have a fixed fuel station, and fill the fuel to the brink, i.e. its visible and remains there for at least a couple of min with the car stationary. this gives me the most accurate average.