My best was a Pan Am discount rental out of Seattle with, I think, National in 1980. Rented for a month with unlimited mileage and drove across country to NC. We were running behind and called from Ohio to extend the rental. They expressed some surprise at our location but sold us another week. We returned the car with 9,600 miles added and I had to correct them when they wrote down 960. We got Green Stamps too! I can see why there might be limits nowadays.