He said 200MPH on the dyno, which eliminates the wind resistance, so I will buy it.
My 68 390 GT had 3.25 gears and a close ratio 4 speed (1st was 2:20 to 1). Tires were BFG Radial Trans Ams G X 60 X 15. The engine had an aggressive cam and lots of power, it would do 6,800 RPM in any gear. I think the speedo was very close to right. It had the optional built in 140 Speedo and 8 grand tach.
I estimated top speed to be 160 MPH, but I never took it up there. Got up to 135 MPH once, and it was not stable.
My current Mustang is stable at 150, but almost impossible to find a place safe to do it, so I don't anymore. My current Mustang is a 5 speed, but it will do that in 4th and the redline is only 6,250 RPM (the engine build will handle more, but the computer shuts it down).
I have checked my current Mustang against 3 different GPS devices - with 295X45X18 tires on the back, the speedo is accurate. With factory tires, the odometer is accurate, but the speed is overstated by about 6%.