I'm guessing here, but I think the 5% figure that Ford uses accounts for the tendency of the "average" driver to ride around on underinflated tires. I'm pretty sure most don't check their tires at all, and they depend on the dealership or whoever changes their oil to do it for them, every 3-4 months.
I was in Discount Tires a few months ago getting a tire rebalanced and overheard some poor old soul being told that he/she (I can't remember which now) was going to have to get 4 new tires because the ones that were on the vehicle were worn beyond safe limits due to poor alignment and no rotation.
- Jack
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