The Fastest!
(top speed for normal & costumized settings)
As most of you think that 911 GT1 is the fastest car in the game, that is not true. True, GT1 achieves 319 km/h, but the fastest car is the car, that is actually the worst car in the game (my opinion). The fastest is 1993 Turbo 3.6 Coupé (964). That's right, that car that is almost imposible to handle, especially on slipery surfaces. But what can I do...
When all upgraded, it has about 690 bhp, just too bad acceleration, pretty good lateral acceleration and a top speed of 320 km/h. Still, this speed is not achievable on the regular parts of tracks, so don't get excited. But...
Enlargable...
... if you play a little with settings on your car, especially with GT1's gear ratios, you can do some amazing things, such as top speed of 358 km/h (in 5th gear!!), which is, achievable on regular track parts!!! Just don't think i got out of that curve in one piece . :) If you do the same with Moby Dick, you'll get "only" 344 km/h.
<-- blast of
out of the subway tunnel 
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And just for some thinking... Absolute top speed in all five Need For Speeds:
For those who had played the first NFS (or still do, like me) - you know what I'm talking about...
For those who hadn't played the first one, read this...
The Absolute top speed ever achieved in NFS, is 341,4 mph - 549,41 km/h
If you think I'm kidding, take a look at THIS replay...! If you want to experience this, and don't know how, follow this instructions: 1. Get The Need for Speed SE (I'm not sure about the original)
2. Win Tournament twice (2x) - it's tougher than you think
3. Take the bonus car (Warrior) and go drive on the City-rally road
I'm pretty sure that you can get even higher speeds with a joystick. Mine was made with a keyboard, and from the records, I saw on the net even much, much better times than my own.