300zx TTs and Z06s must have pretty good downforce without wings because GSX and I cruised at 150 for about 12 miles and did a roll from 40-165/70 and both cars felt glued to the ground. Once you are over 130 or so it didnt feel any different to me as far as the speed. The mental thought of going that fast after you have done it is scarier then actually doing it. You definetly need alot of room. When I drove ZphatZs car and had to slow down fairly quickly from 140 the car got a little happy but nothing too crazy. The bottom line is to do it in the safest enviroment possible but preferably a track because there is no time to react at those speeds should something come up in your lane or field of view.