I know no one here seem to care much about rear lift, but if you're interested in car aero...
Our W212 Mercedes 2.1 diesel twin turbo. Excellent mileage (6.1 litres / 100 km over 40,000 km - that's 39 US mpg - for a large car that's quite quick at the top-end), but crap ride (the suspension has sunk a bit I think, and it's quite wandery in crosswinds and at speed).
Testing showed it has zero front lift – the front ride height does not vary at all with speed. However, at the rear, the ride height increases by over 10mm at high speed, indicating the rear lift is occurring.
Testing was with this:
...and this:
It's an ultrasonic sensor working with an Arduino and software I wrote in XOD. I don't think I have mentioned it here before because I wasn't happy with the accuracy versus direct sensing of suspension ride height - but the results are indicative.
Pressure testing supports the idea of lots of rear lift:
So, what to do about it?
I tried a corfboard rear spoiler at 90mm height:
It increased pressures at one spot on the rear boot (trunk) from zero to +40 Pascals average.
I then tried a smaller spoiler (45mm height)...
...and it dropped the increase in rear decklid pressure increase only marginally - to +35 Pascals average. (All at only 80 km/h - 50 mph).
Now I am thinking of small aftermarket spoilers like this...
...or bigger ones like this...
I haven't done any throttle stop testing yet (or, alternatively, mileage over 1 km at high speed), but I am leaning towards a smaller spoiler.
Unfortunately, my wife (whose car it is) isn't at all interested in contributing $ to a spoiler. So I might have to show a mileage increase as well.