All cars have what is called a "road load power" curve.
It's basiclly a function of speed (or RPM * some conversion factor)cubed.
All engines in these cars have a horsepower curve that is a function of RPM.
The point where these two curve cross eachother is the car's potential maximum speed, period.
To actually achieve this top speed, the power curve must be crossed at it's maximum, this is why cars have gears.
The most effective way to reduce road load power is to lessen the cars air drag, either by decreasing the frontal area or decreasing the drag coefficient.
This means removing side mirrors, trim, making sure the car's roofline is level with the ground, ect.