raph - everytime you post it gets funnier and funnier. 225HP = 150 mph in a stock Stang. That's a good one.
I hearby announce that Raph wins the StangNet monthly "I only open my mouth to change feet" tee shirt contest. It, like the check, is in the mail!
The power required to increase speed and overcome aerodynamic resistance increases with the cube of the % increase in speed.
So, here's how it works. Let's say a stock stang puts down 190HP at the rear wheels and with the 2.73 gears is capable of about 135-140 mph (that's about right for a stock car, even though raph's will go 150

). So, for the purpose of the calculations, let's assume 135 mph. 165 mph is 22% faster - or a factor of 1.22 faster. Cube that and you get (1.22X1.22X1.22=1.83) 1.83. It will require 1.83 times as much rear wheel HP to push the car from 135 to 165 -- 190 X 1.83 = 347HP at the wheels, or about 420HP at the crank. So, to get another 30 mph of top speed requires almost twice as much crank HP as the stock engine. This assumes you've geared the car to hit top speed at the power peak in one of the top two gears.
If you wanted to hit 200 mph based on the same math, it would take 600 rwhp to do so.
It's not a perfect approximation, but it is close. And it makes some sense. The new Corvettes which are MUCH more aerodynamic than a fox body Stang, seem to regularly trip the rollers at around 350-360 rwhp - and they need that to hit 175-180 mph.
Raph - I suspect you've confused the numbers some calculation spits out with what will actually happen when you try to force the car through the air at those speeds. No doubt the rpm/gear calculation says 150 mph in 4th gear is possible -- but in a stock Stang, the HP is simply not available to make that happen. It will run into a wall of air somewhere around 135-140. You might do a bit better with a shorter gear - as it would allow the engine to pull further up into the power band at speed.