First off, I'll talk about in-track topspeed.
I've gotten 1300+mph with the GT-One car in MaxSpeed test.
And I've gotten 1800+mph with GT1 again in 2player mode by running into the player midway through the turn so he comes out of the test course turn @ 260 or something.
Another way you can get some sick speed is drafting in 2 player mode. Just start off normally in test course, have one player stay infront of the other perfectly and when you come out of the corner, try to have player 2 stay behind player 1.
If you have 2nd place boost on high then the 2nd player will keep trying to go faster to catch up to player 1. And player2 is drafting player 1 so he goes even more faster. This is another way to break 1500+mph, gotta use the replay to see exactly how fast tho...
Now on to the 2.14 blah blah numbers everyone is so confused about. GT3 uses a certain type of variable to store its MPH/KPH value. It's called LONG and its maximum value is right around 2.14 billion. It's obvious GT3 is just overflowing its MPH and its odometer, thats why they both stop at that speed and thats why it doesn't matter if its in miles or kilo.
Just thought id be a dork for a minute and explain some shiz