Well, there are several entries you can change, but as said the maxspeed is the most important.
class RAS_Racer2 : Mini
{
maxSpeed=250;
displayName="RAS_Racer2";
nameSound="car";
accuracy=0.50;
armor=200;
unitInfoType=UnitInfoShip;
terrainCoef=1.05;
};
this is the config of a tuned mini I made for myself. It runs like a maniac: Topspeed 192 kph, 0-100 in 5 seconds when I remember right. It also doesn't really slow down offroad.
BTW, there is a speed limit for cars, which depends on their weights (if I remember correctly).