It happens that Ed Sirett formulated :
IIRC about 20 years ago, 30 mph, was about evens between the air drag and
other resistances (mainly tyres). These days the shapes are more slippery
so 40mph might be the balance point.

It all depends on what you mean by 'significant' and 'dominant'
If 40 mph was the balance between aero drag and other friction then
At 60mph aero drag is about 70% of total which I call significant.

I've done a bit of caravan towing rough research using the fuelling
computer and given still conditions and fast roads, my most economical
speed is around the 50 to 55mph mark - surprisingly, because you would
think the poor aerodynamics would make it very uneconomical. When in no
particular hurry, I can manage economy figures only slightly worse than
when not towing.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk