You said yours did 50mpg at 147mph.
In link another owner says his does 45-48mpg when cruising.
I asked if cruising speed was 147mph.
As usually cruising speed is 70-80mph, and this speed will usually achieve better mpg than being flat out at 147mph.
Maybe these cars return their best mpg when absolutely flat out battling against wind resistence, I've not driven one