The reason your car has a top speed less than what your gearing says you could be at is drag. Your car simply doesn't have enough power to overcome drag.
Drag is the force that pushes against an object as it moves through a gas/liquid... Example: Stick your hand out of the window while going down the freeway (as if you were giving someone a "hi-5").. Notice you have to put some effort to keep your hand still as the wind pushes against it. So we've established that it takes effort to move your hand through the air.
Its the same for your car. It has alot more air to push out of the way, so it takes much more effort to push through the air. So much force is involved that typically it is aerodynamics that will limit most car's speeds as opposed to gearing. If you want to compare other car's aerodynamics, the proper terminology is "coefficient of drag", or a 'Cd'. My old Rx-7 had a Cd of about .26, which by the way is
really aerodynamic, and off the top of my head, I'd estimate the Cd of a '98 2.5RS to be about .35ish. Which is about the aerodynamics of a brick in comparison.
Another way to increase top speed is to reduce the Cd of your car. Some ways are more effective than others. Under body covers help a fair amount, removing the side mirrors help. Rims that do not have spokes (think flat discs) help, as partially covering/blocking the front grill. Tires that are more narrow will also help a great deal, as well as removing mud flaps. And believe it or not, lowering your car will help as well.
