I found How come the ground effect has such a different outcome for wings and cars? and: I am not allowed to comment as a new user because of some weird rule and I do not know enough to answer the question as I searched the web to find the answer to this.
Everyone is explaining that the two types of "ground effects" have the same name but are two entirely different phenomenons, but no one seems to have answered the question properly, at least that I can understand. It is all well and good saying they are different effects for different purposes but that does not explain why the two opposite effects occur in such similar conditions. I was originally searching for the answer to this question strictly in the context of car design - why the car-context ground-effect sucks the car down using the venturi effect yet having a full teardrop shaped car is meant to be impractical for the reason of creating a lifting air cushion underneath, though someone may think that the curve towards the ground would act as a venturi and suck the car down. The question is: "why do the opposite effects happen in such similar conditions". Venturi effect or air cushion? What decides which occurs?
I think aeroalias' answer sort of explained it but not in a clear and direct way; venturi effect occurs until the venturi is so extreme that the flow is "choked" and the venturi effect ceases to operate and you just have an air cushion? Is that the answer?
edit: This might make it clearer as to what I am asking though it is mainly a rephrasing of what I have already asked:
Why isn't an air cushion formed underneath a car (like with the aircraft) when the underside is curved towards the ground? Does the venturi effect occur when the curve is shallow but an air cushion form when it is steeper and flow is "choked"? I think I understand the basic theory of the car ground effect but what causes the aircraft ground effect and what conditions determine whether upward or downward force is produced?