I book on the AMEX PLAT code, which, as described by AMEX, gives me an upgrade when available. Apparently Hertz/AMEX normalized this recently as Hertz 5-star status. In ATL this weekend, they have a huge inventory - the clerk said I'm 5-star, and though I had reserved a full size car, I was not eligible for an upgrade as 5-star status does not allow for upgrades into premium.

In RNO several times this past fall I was upgraded from F to premium - I wonder if this is not going to happen any longer.