Ok, now I`m confused. If I could get 50miles/gallon in ANY car I`ve ever owned (ok, my small Opel diesel ute could almost manage it) without towing anything, I`d be really impressed.
50 miles = 80,5km.
1 US Gallon = 3,785litres.
1 British Gallon = 4,546litres.

This translates to a fuel consumption of 5,7litres/100km, my diesel did 6,2l/100km on a very good day. My current 2.0litre petrol engined car gets about 10l/100km without towing, most new cars beat that but only by a small margin, say 7-8l/100km.

I know you have no hills to tow up, Wouter, but are your figures even remotely possible ?