Chris, I usually rent with Hertz, Avis, or National and I've never been asked to pay more for a larger vehicle when the car class I reserved was not available at pick-up.

I did have a situation last October with National where my flight arrived late and they were all out of intermediate size cars. The only thing available was a huge Chevy Blazer. I didn't really want that, but it was after midnight and I still had a 1/2 hour drive to my hotel so I took it. The thing was a real gas guzzler so I ended up paying more for fuel than I would have with an intermediate car. In fact, I paid more to fill the thing than I did for the entire rental! So even though I wasn't charged more by National for the larger vehicle, it still made a dent in my wallet.

I wish the car rental companies would set aside the proper size cars for those with reservations and offer the larger vehicles to walk-ups.