I suspect mine is too loose as it will occasionally slip mid-stride causing my landing to slip forward a bit. Details would be helpful.
Update: If it's relevant, it's a Life Fitness T5-0 treadmill.
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Start by turning on the machine and pressing “Start.” Adjust the speed to the lowest setting. Let the belt rotate a few times. If the running belt starts to shift toward one side, loosen the rear roller bolt on the opposite side ¼ turn clockwise. For example, if the running belt begins to shift to the left, loosen (turn counter clockwise) the right roller bolt a ¼ of a turn. The key to this is patience as you must wait five revolutions of the running belt between each adjustment. That will ensure you do not over correct the belt.
If you need more information read here: http://www.visionfitness.com/blog/how-to-align-and-tension-your-treadmill-running-belt/