[aerohead]

At posted speed limits,you can't legally drive faster than 85-mph (between Austin,Texas and San Antonio,Tx.) in the USA.
In 1969,a GM Pontiac GTO "Judge" was road tested in the Nevada desert.
At 115-mph,the car,without the rear spoiler,generated 105-lbs front lift,and 250-lbs rear lift.
Those testing the car felt that due to the car's weight,none of this lift was an issue for stability.
Since these forces vary as the square of the velocity,even at 85-mph across the 41-mile stretch in Texas,this lift would be virtually meaningless.
Hopped-up,and at El Mirage or Bonneville,yes,but not in everyday highway commuting.