He has send me some pictures:

He uses 17.5 Liters on every 100 Km. 

When you do not drive the amount of kilometres is zero. and you will get an infinite number by the division of zero error. When this happens the display will show some dashes. It would be nicer to switch to fuel/hour in these situations. However, nobody has made that yet...

The intro word.

The date. This a standard function of the display and has nothing to do with the module.

One important remark he made is that he needed to disconnect the resistor connected to Pin 7 of the PIC16F84. The exact location is denoted by the cross in the lower left corner of this picture. We have not yet figured out why this is nescessary.

The front side of the experimental board.

The back side of the board.

I would like to thank Rado for the pictures and the information.