Speedo v odo

MY VW Passat R36 and TomTom GPS show a difference of almost 7km/h at 110km/h, the car's speedo showing the higher speed. This works on a sliding scale, the VW is about 2-3km/h faster than the TomTom at 50km/h and 6km/h faster at 100km/h. I have also found that when using the odometer tests on highways, my car shows I've travelled one kilometre when I've actually only travelled about 950m. Am I right in saying that if my car believes it's travelling at 100km/h when it's actually travelling at 95km/h, that the odometer clicks over for 100km travelled when I've actually travelled 95km? If this is the case and I have a 70-litre petrol tank, if my odometer shows I have travelled 700km on a tank of petrol my fuel economy will be 10 litres for 100km. If I've actually only travelled 665km my real economy figures would be closer to 11 litres/100km.