[sick472]

Try comparing a Ford cap to your chevy cap and see if there is a noticeable difference where they snap on. The only thing I can think of besides that is temperature changes over night that would cause the cap to shrink up more than the rims and the added pressure at the fit is causing them to squeeze themselves off. Do they go on easy? Maybe they are too loose, or if they are too tight, this would aid in my shrinking theory. FYI, when metal cools down, it shrinks and a cap may shrink more than a wheel causing an even tighter fit. This could work for the snugness of the fit or against it if the nubs on the wheels are angled such that they allow the cap to push itself off.

Quite a mystery!