Fuel overcharge

I hired a car from First car rental on 12 June 2015. Firstly the website charges and the real charges when you arrive at the airport are not the same. I have read other complaints of a similar nature and can attest to that. On returning the vehicle I filled the car at the BP station and calculated the fuel consumption to be 15.5km/l (or 6.5L/100km). This is in line with vehicle specs for a Polo Trendline on the web. When I received the final bill an addition charge of R144.81 for 11.2 L was added. This meant that the vehicle uses 11.3 km/L (or 8.85L/100km). I drive a 3L Isuzu 4x4 and that is what I get in town! I complained and the person handling the complaint even had the audacity to state that they work on a consumption of 10km/L as \this is what was given to us by the manufacturers bearing in mind whether the vehicle was driven in urban or rural conditions or highways too."If VW claims that"