I go by what is recommended on the sidewalls of the KDW's. 40psi for predominately city driving and 42psi for highway. If you run these 50 series tires underinflated you run the risk of bending a rim (I speak from experience from this past winter - I used to run 32-34psi). I was also told by my Ford dealer service manager that underinflating the KDW's causes cupping of the tread. Although my tires are already trashed (except for the one replaced by my insurance company) and due for replacement, I run the higher pressure for the health of my rims.
