Hmm. I haven't tried this in NFS 6, I just made cars more realistic there. In NFS 3 when you accelerated to such a speed that the computations of the engine went wrong, you flew out of the track and the speedometer read 1 billion something, though this was not done by accelerating continuously there from standstill.
The other one: I've downloaded 800+ cars for NFS 3. It's somewhat interesting because I didn't like it at the beginning, the physics were awful. I've made an Excel chart in which if you enter the data of a real-world car, it turned them into NFS 3 data. It was easily more realistic than the original. Doing this was easier in NFS 6, you didn't need conversions there.