There shouldn't be any hard limit on the speed of the vehicles. You just need to modify the right parameters. Here are the main modifiable parameters that will affect speed/acceleration:
TorqueCurve = For an input RPM value you specify an output Torque value. The higher the torque value, the more power the engine has at that RPM range.
GearRatios = 0 is reverse, 1-4 are forward. Set your gear ratios similar to a real car (trading quick accelleration for top speed, etc).
TransRatio = This is the transmission gear ratio. Just like a real car, higher gears will give faster acceleration, but lower top speed.
ChangeUpPoint/ChangeDownPoint = This is the RPM value that the transmission will switch up/down gears. You want to switch up after the engine has reached its power curve.
VehicleMass = This is the "weight" of the vehicle. It doesn't really map to any real world value unfortunately. You just have to play with it until your vehicle "feels" right. Just remember, the higher this number, the more power it will take to accelerate your vehicle and the more power it will take to reach your top speed.
I hope this helps. I'm looking forward to seeing your stuff. On a side note, Unwheel for UT2k4 did a lot of car racing stuff, and it has the exact same vehicle physics codebase. You might want to see if you can get in contact with the creator of Unwheel and see if he/she will share thier source code with you so you can use it as a starting point.