Hear what you're saying, you're right, of course; but there's turbulent air coming off the back of any moving vehicle. Best figures I can find is Omega saloons at 0.29Cd, the estate 0.32Cd, (though something tells me those figures aren't
quite right
)
So roughly there's 10% difference between the saloon and estate versions in Cd, with the effective air pressure/force acting on the front quadrupling with every doubling in speed. Or, if there's that theoretical 5kgs at 30mph, which would be 5.5kg on the estate, that's 6
6 kg at 90mph, or 4kg at 60mph. Or an effective, theoretical 3kg extra 'weight'/force at 50mph.
Not disagreeing with you, because, of course air resistance
is acting on moving objects all the time, otherwise the famous demonstration of dropping a hammer and a feather on the moon wouldn't have gone the way physics professors said/knew it would. Definitely, weight affects acceleration at lower speeds, aero at higher. I'm just going by my old Aero Engineer / Lecturer's feelings at what speed air resistance starts to become 'substantial' but of course one person's idea of 'substantial' vs another's can vary massively, it's opinion. Like I say, can't say you're wrong, as it's a grey area, there is no black and white line where air resistance doesn't affect things. (well, being
stationary, I suppose, haha!)
Don't know if the saloons and estates had different diffs to counteract things, but the top speed difference is only between 2 and 4% between saloons and estate, too.