I try to keep about .020 between the rail and the magnet. Traction magnet strength should also be considered. Some are so strong they will slow the car. Scale auto makes a neo traction magnet for their G3 that is so strong that it will kill your speed thru the traps. I have lost about 15 MPH with these on an Unlimited. You could easily run these on the ceiling if you wanted. They also tend to bend the chassis if it is not stiff enough causing the magnet to drag on the rail. Magnet size also makes a difference. If you have a gauss meter a tiny neo magnet will give the same reading as a large one. The mass of the magnet makes a difference.