The top speed can also be limited by having the available HP to overcome the aerodynamic drag of the vehicle and frontal area. You may top out on power before ever reaching that top speed - thus this is the only time the engine might come into the equation (based solely on the higher available HP if needed).
That is VERY true. Using the formula you could attain a theoretical speed, but as the speed increases, you need far more horsepower, far more than you would think, to overcome aerodynamic forces.
I recall reading once where a Bonneville team made it to 190 mph in an older A-Body car, with around 500 hp. They needed something like another 150-200 hp to get another 10 mph.
Look at the new Dodge Challenger, a car with a good amount of hp, overdrive trans, and is very aerodynamic compared to our older cars. They can barely attain 140 mph. Back in the 90's, the then-new 1998 Ram Air Trans Am's were capable of 150 mph, and that was a slippery body, with 300 net hp, 6-speed overdrive trans, and 2.73 gear.
Most of the older cars (68-72 GTO, 67-69 Firebird) were capable of around 125 mph. Some could go higher, especially those cars that had engines capable of higher revs than that of a Pontiac V8.
