I have in my profile (and also in every individual reservation) request for specific cars. You can request some specific cars, but these are normally only the few that Hertz admits to carrying, not the stragglers they carry anyway.

For example, I really like the Chevy MonteCarlo, even more than the Mustang, and I requested that. However, I couldn't request a MonteCarlo, the best they could do was request a Chevy. In the end I got a Toyota !?!

Today I reserved a Ford Focus and had the agent specifically put in 'Do not upgrade (I HATE CONTOURS) and Ford Focus only' I'll see what happens!