On another note recent experiments have made me realise i lose 15mpg with a tow bar mounted rack with 2 bikes on
Is it also that you had more ‘load’ in the car?
Only as a friend who had a new 520d was getting +50mpg when on his own averaging 50-60mph, and then on a run to Cannock with 3 bikes on a towbar rack plus 3 blokes and gear it was struggling to do 35mpg at 70mph.
But the same 3 bikes on a towbar rack on my older 535i made no difference to the mpg, at any speed.
And I’d imagine that bikes on the roof would make far more difference, as just the roofbars use to knock my petrol Vectra from 33mpg to 30mpg.