First, this is against the TOS.

Second, if you happened to find yourself on an officially sanctioned race track going 70 miles per hour, third gear should be fine. The only way to tell would be to run third gear to redline on this officially sanctioned race track and find out the speed indicated at redline. You can downshift into third to within about 5-10mph of the third gear max speed.

Example: My third gear redlines at 90mph. In fifth gear at 80mph, I can downshift into third if needed. I use it for passing accelleration though, not for racing. For reference, I have a 1.366 third and a 4.10 final.