There you go. I too wish you well in going after the contractor. I'd agree that the base, the mix, and the weather conditions play an important roll (all of them really optimizing the finished product.) Concrete is poured in the winter just fine too.
What you might want to do in the meantime is get a few quotes to replace the driveway. Ask what PSI they're using. Tell them what you're gonna be driving on it too. I have 4400 psi curbing and it's been wacked a few times by PENN DOT snow plows, and survived. If different contractors are quoting different PSI's, ask why.