The computer is probably using the instataneous fuel usage/min and Ks per min to work out the Ks per litre. You could always do as I do, and use the old paper record system (spread sheet actually)
If you want, I can post my spread sheet, all you havw to do is put in your distace traveled, the date and the cost of the fuel. The spread sheet will give you average cost, fuel usage, distance traveled, MPG (US and Imperial), Ks/L, L/100 Ks, Ks/$ and expected range from full tank and to the reserve indication