I prefer not to get into a discussion regarding the ethical considerations about high speed runs on public roads. Clearly they are best done on closed courses where public safety isn't a consideration. That said, there are no doubt some public roads where high speeds can be done without endangering anyone besides yourself. With that disclaimer out of the way here are some numbers to consider. All of these numbers unless otherwise stated are computed using a 27 inch diameter tire [ standard GT size ] and standard 3.31 rear end gears. I am also assuming that we are talking about a MT82 manual transmission and 5th. gear [ 1:1 ]. Assuming stock Gen 1 connecting rods and stock cams and valve springs I think 7,200 rpm is the maximum safe rpm limit. At 7,200 rpm top speed is 174.7 mph assuming that aerodynamic drag will allow it. Keep in mind that both aerodynamic down force [ which we don't have ] and aerodynamic drag of which we have a lot increase at the square of speed. Basically at these speeds it takes a big increase in power to get another 5 mph unless you find a way to reduce drag. My understanding is that the factory top speed limiter is set at 145 mph. With 3.31 gears 145 mph is 5,975 rpm, with 3.55's it's 6,408 rpm and with 3.73's it's 6,733 rpm [close to the factory rpm limit]. With 3.55 gears top speed at 7,200 rpm is 163 mph. With 3.73's and 7,200 rpm it's 155 mph. One last point. I think the speedometers/odometers on modern cars are much more accurate than the mechanically driven ones on older cars, but they still aren't perfectly accurate and the error tends to get worse as speed increases so don't take a high speed reading as being perfectly accurate.