'ZR' rated tires have a 'maximum' speed of 186, or more properly, they are capable of going at that rate of speed before coming apart, though that isn't likely.
Our cars are indeed governed though just not mechanically. The ECU electronically prevents our cars from exceeding 157mph.
I've had both cars to the limiter & it's a funny feeling: it just won't go any faster. One just gets there faster than the other !
As to how fast it can go, no one really knows. Steve Hellums has had his SRT at 180. I'd LOVE to have some member some time get his Xfire out on the Utah flats & see just how fast these things WILL go. I have no doubts that the Crossfire could be in the 200mph Club.
If it can beat the air resistance as Kurtis mentions...........