Are you saying..
If you use any additional custom tracks your car will not go any faster than 17kmh or get out of first gear....
I have never heard of that happening before... i have nearly all of the additional tracks, all the cars seem to work exactly as they should..
The only reason the car should be stuck in first gear is if you have the gearbox set to manual gears and you forget to change to second..
There is no reason a car would behave different on custom tracks to tracks that are included with the game. They all have the same stuff in them, like the start line, pit boxes, Even if the game though the entire track was a pit lane, it should still let you drive at about 45kmh.
I would guess and say you have maybe accidentally changed a setting in the game without noticing. Double check everything in your Options ( especially Controller Options ) also when you pick a car and track to play on, you see the "Summery" where you get to turn on or off some game settings like traction control or Automatic gears... Make sure those are set to what you normally use.
Other than that i really have no idea what could be wrong. all i can say is, the tracks are not the problem as lots of people use them without having issues, which would suggest it is specific to your copy of the game.
There is one obvious thing you could try. Restart your computer, a setting or a file could have got messed up, restarting might fix that (it certainly won't do any harm)
