[MetroMPG]

Whereas my car's unaeromodded flat-out top speed is 100 mph. (~50 hp. )

There's no way I'd be attempting to drive 50 mph into a 50 mph headwind - accelerator floored in 3rd or 4th gear or whatever it would take. I never need to drive anywhere that badly.

Of course if someone else is planning to mod a car to drive in those conditions, then of course higher apparent wind speeds need to be taken into account and/or tested. I only drove this proof-of-concept tail at road speeds up to 100 km/h, in let's say ~15 km/h winds (guess).