I rented a car the last week of December 2010. On the next to last day I got a flat. It was too late for me to take it to a rental place by the next morning, so I changed the tire. When I dropped it off I wasn't told whether or not I would get charged. The attendant actually tried to print a coupon for me because of my inconvenience but was unable to. That implied to me that I wasn't going to be charged. Now over six weeks later I've gotten a bill from Avis for $143 for replacement of the tire. I will never use Avis again.
From now on when I rent from anyone, I'll be checking the conditions of the tires and whether they're low on air.