What if I don't want the car rental upgrade?

When I book a rental car, I don't do it with an upgrade in mind. I prefer a smaller car because I like the easier parking and the better gas mileage, plus I usually don't need a ton of extra space. But, on occasion, I'm offered a free upgrade to a larger car. So how do you avoid an upgrade if you don't want one? I wrote to a few car rental companies to see what they'd suggest.
Avis
said that agents offer free upgrades when the car size a customer reserved isn't available, and that if the customer strongly prefers the originally reserved car class, to tell the agent so. Then arrangements can be made to exchange the car during the rental period as soon as possible.
Enterprise
responded that if it didn't have the correct car at a location, it would look to neighboring rental locations to find the vehicle.
The folks over at
Dollar
and
Thrifty
said that, in the case the correct class of car wasn't available, the customer's three options would be to wait until one was returned, to see if the rental agent can arrange transportation to your hotel and would be willing to deliver the proper vehicle when it was available, or to try to rent through another agency. Also, since weekends are the busiest rental period, by picking up a car on Monday through Thursday you increase your likelihood of getting the car you want.
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