Heh, you are right, it is physics. While gravity applies to cars, at these speeds aerodynamics take over similar to how they take over for airplanes. These are landing/takeoff speeds for most aircraft you know, right? We can really make a scientific study of these physics if you want but here's some brief introduction:Errr... Really??
It's physics... Newton's laws of Gravity don't apply to cars huh?! Your trying to say that two cars created equally in every way, one weighing a thousand pounds more than the other will have the same top speed? Really?
I'll quote wikipedia because their description is very accurate: Drag Force Equation
You (or someone else above) brought up the Bugatti Veyron and this little thread gives a lot of insight into how terrible that particular vehicle is to look at relative to this discussion of top speed. That vehicle is a brute force attack on top speed, but even the Bugatti uses sails to break from top speed as flaps pop up when you're trying to break, other cars do this. Other vehicles, with much less power, are able to top out nearly as high as a Veyron because they are better engineered for aerodynamics. More power helps but aero is the key.
A large part of aerodynamic design in cars is to actually creat downforce. This is something which your higher weight vehicle actually accomplishes from the get-go before aerodynamics kick inf or real. The point there is to plant the wheels down and to NOT turn into an airplane at the first bump.
I'll give you another example, take a motorcycle and a Corvette with similar power ratings. It is fairly easy to turbocharge a Hayabusa to 500 hp. The power to weight ratio is absolutely astounding on motorcycles. This is why they are so quick off the line if you can keep the front wheel down, about 8 years ago the
You are right about one thing, in the real world weight matters due to rolling friction and etc, you'll find out that those differences are very negligent when you approach true aerodynamically limited top speed. In other words, a good tail wind will have much bigger bearing on your top speed rather than how many passengers you fit into your car.
Hope that helps a bit in the discussion and I haven't offended anyone with my physics.