That sounds like a nifty business!
Here’s how I would use Add-Time. I assume most of your reservations are booked over the phone or from your website, while you are out driving customers to the local wineries. So the iPhone App will be your main control center that is synced with your calendar. You can use your iPhone to setup appointments when people call, and you will be instantly notified when someone makes a reservation from your website. Everything will always be up-to-date.
I am guessing that each car and driver is booked for a tour lasting a specific time, like 4 hours. If your times vary a lot, just setup as many different times slots as you need to cover every possibility. The drivers are your staff, but they are not your biggest selling point. So instead I would enter the cars as your staff. That way, when someone is booking online they can choose the 1930 Packard Eight or the 1937 Cadillac Twelve, as an example. With is setup, each car will have its own appointment calendar.