Top speed has everything to do with the mass of your car. In order to reach top speed, you need to generate enough force to accelerate the mass of your vehicle to that velocity. Force = Mass x Acceleration.
Let's say that your engine can generate a constant force, which it can't do. The mass of your vehicle will remain constant. As the velocity of your vehicle increases, the force of drag on your vehicle will increase (quadruples for every time the velocity doubles). The force of drag can be subtracted from the force your engine is able to generate and then you can divide it by your mass to determine the rate of acceleration. Your acceleration will constantly decrease until, for all intents and purposes, it reaches zero.
Now factor in the fact that engines don't generate a constant force and force drops off significantly at higher RPMs.
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351W 89 Mustang GT Convertible
[This message has been edited by jimberg (edited 07-23-2001).]