A further complexity to add to the equation is that when going for top speed, you have no choice about the engine revs.
For example, say your engine produced maxiumum power of 350 bhp at 6,500 rpm, and 6,500 rpm was geared so that it equalled 150 mph (these are made up figures, don't necessarily match a skyline).
Accelerate to 175 mph. The engine is now doing 7,500 rpm, and will be considerably off the power, since power peaked a while ago. And in practise, there may not be enough power at 7,500 rpm to maintain 175 mph, so you'd never have got that far even though your calculations from Guy's post said that you would be able to.
I really hope that make sense. I don't think it does.