That's a good question and I really don't know. To be perfectly honest, though, in my head, at least, any number less than maybe 10 fast food restaurants to each car dealership is low on the fast food restaurant side I reckon... but again, that's just my gut feeling about the ratio.
Except, a single McDonalds can serve ~5 orders a minute. Yet, a car dealership that can sell 2 cars a day is doing Ok. Let's assume 2 cars a day over 5 years = 3,650people before you get repeat customers. (Average salesman sells 7 cars a month * 8 sales guys.)
