With my '64 Custom/427, using 8.20-15 tires on the back and a 3.50 rear end, third gear was good to more than the speedo (120) would go. The '64 dash was laid-out in such a way that the needle on the speedo would disappear to the right and briefly show in the red idiot light for the 'gen'. I had several experiences of running a corvette or a 409 and downshifting to 3rd at 80. I'd need to shift to fourth after the needle disappeared. I measured the tire radius on the rear and did the calculations. I don't remember the exact numbers but I could pull six grand plus in top gear. And the offending chibrolay would be nothing but a dot in the rear-view. (The front end became a bit light above 130.
KS