Hmm, I'll have to dig out my old aerospace engineering text book to look up the formulas for calculating drag. Top speed and horsepower are most definitely related.
The 427 Cobra was NOT designed for 1/4 mile runs. It was a road racing car that defeated Ferarris.
Speedometers are indeed inaccurate. They're off by a percentage, too, which means that the faster you go, the more they're off. They're sometimes off in a favorable direction, though, meaning that you may see 130 but are going 135.
Unit, the orange line on your tach is the overdrive gear red line. I think it's 4900 rpm. That's a 220mph top end.
Steeda had one of their Mustangs approaching 200 on the track. The body panels started peeling away.