A big part is your altitude. At 4500+ ft your probbably 1 to 2 seconds slower than if you were running at sea level.

The cars weight, gearing, and traction (60' time) also make a big difference. If you can launch the car at 5,000+ RPM and get it to hook you could reduce 60' times by about 1/2 second compared to launching at low rpm because the car won't hook up.