Avis -- number 2 in the industry and Trying Harder -- once had a policy that cars were replaced every 6 months. They introduced a marketing policy in early 1970 as a show of force and to demonstrate that their cars were new:
"If you were rented a 1970 Plymouth with more than 19,000 miles, you could keep the car."
We know that Avis had no desire to give away free cars and given some of the vehicles of the early 1970s, we're not certain whether this was just an absurd offer or a punishment.
Although the premise of the commercial was completely implausible, this happens to be a favorite among the AutoSlash team. Avis didn't have "Select & Go" for their Avis Preferred members in 1970 -- Wizard wasn't introduced until 1972 and Select & Go came out in 2012. A renter couldn't simply run around the rental car company's back lot, determine which fine Plymouth automobile was going to be rented, and then run back to the counter for a free car. Yet one could always hope and the Federal Trade Commission has never really worried about puffery!
And while it doesn't compare with a chance to drive away with a free car, Avis did release an offer for free flights a few decades later.