I usually say miles an hour in conversation, but I'll say 'per' if I'm referring to a speedometer I think.
Murpheeee, funny you ask about "What's your ETA?" I picked up that phrase recently from my boyfriend, who says he picked it up from the Lincoln Towncar chauffeurs in the city. After you give them your destination, they'll tell you formally what your ETA is going to be, to be more professional and courteous than cabbies are. Now that my bf drives me around a lot, I end up asking him often "what's your ETA?" too!