A tip I learned here - can't remember from who - is to leave about a half inch between the tire and the front wheel stop when loading your car. Then when you cinch the straps tight it will pull the wheel tight against the front stop. I Have been doing this for about 6 mos and no longer have to retighten my straps. I check them but they are always tight. I used to pull the car tight up against the front stop and I think what happens is that it leave less room to tighten the straps and since there is always play/rocking in the P gear. Using the straps to pull it tight against the front stop instead of parking it against the front stop eliminates the play that works the straps loose.