How accurate are golf simulators?

I am 19 and have been playing golf ever since i can remember. I recently went to the PGA store near me and hit on the simulator today with the 8.5, 909 D3 Titleist driver with a stiff shaft. Now i can crush a drive...but it read my club head speed at 114, and the distance at 326 in the air, with a 340 total with the roll. Obviously, this cannot be accurate hitting a 340 drive...but exactly how accurate is it?