The cheap rental rates with which you are presented upon making a reservation could vary depending on the rental company's mileage criteria. For example, you could be presented with one of the following:
Some companies will rent on an hourly basis. Independent local companies are normally more relaxed about these types of short-term rentals.
Are you planning to use the car for 2 or 3 days? Then getting a daily rate could be your best option.
Weekly rates are best for when you want to rent for 6 days or more.
The best way to find cheap car rental deals is to check competitive rates, since rates are constantly changing due to supply and demand. Car Rental Express shows several rental agencies in most major cities, so you can shop around. That being said, book as far in advance as you can (at least a week) because car rental companies book up quickly, especially during peak travel periods.
If you have to change your plans, then act accordingly so you do not incur extra charges. Some companies have cancellation policies where charges are added if you do not cancel within a certain time period (this is normally 24 to 48 hours).