I got it from. It's when the car has a tendency to follow the grooves (ruts) in the pavement from years of traffic, especially semis.
Around here there are still alot of old roads that the surface literally appears to be grooved from all the semis. Since the ruts are wider than th etrack of a normal car or a Z, any car that rut-steers will seem to wander on its own on the pavement between the two grooves. On bad roads its annoying as hell. Luke told me that putting a tire on a wheel that would result in stretching it a bit, would make this more pronounced and it makes sense since you have alot less sidewall deflection to work with. But it also gives you "crisper" turn-in feel, which is more important to me since I like to go do track events every now and then.
