2,8l Cabrio needs over 17,5l at 50 km ????

I hope you can help me further, I recently bought an Audi Cabriolet with the 2.8l V6 machine and have now asked a few questions about my fuel consumption. In the city I actually never drive over 2200 revolutions and would also say that I need there under 10l/ 100km. Was now the days twice on the highway and had it run properly. The result after about 50km goods a top speed of about 220km/h and a fuel consumption of 17.5 l after just 50km. normal, 17,5l after 50km full throttle ? Would I really be happy about some honest answers !