Incidentally, for anyone who is actually interested in facts rather than supposition and erroneous theories, take the Porsche Taycan as an example of a modern fastback - and let's look at the topic of aero lift.
In profile:
CLr (coefficient of lift, rear) : 0.16
(That figure is from a presentation Dr Thomas Wolf of Porsche was kind enough to send me - rear spoiler not raised.)
Frontal area 2.33m^2.
Calculated rear lift at 100 km/h - 17kg
At 200 km/h - 70kg
At 260 km/h - 119kg (that's 262lb!)
No wonder they fitted an automatically deploying rear spoiler....
All fastback shapes with attached flow develop lots of low pressure over the upper curves. That can only be offset by:
1) rear spoiler (usually only a partial offset of lift), and/or
2) very effective underfloor aero (can completely cancel lift)
But as I keep saying, there's absolutely no need to guess or use only factory coefficients. Just measure the actual, real, aerodynamic panel pressures on your car on the road.
My Insight's measured pressures (my car runs very effective undertrays) - Pa at 80 km/h:
Note the low pressures all the way across those upper curves. It's just what happens with attached flow and these shapes.
So the next time that 'Aerohead' states his completely wrong notion about fastback shapes having no lift, can we actually ask for some evidence - real evidence - and not just pretend the emperor has clothes... and that we should all be politely agreeing with claptrap?