Car rental companies generally use one of three different methods of giving you miles, depending on the airline and the car rental company:

1. A fixed amount (usually 250) per rental.
2. A fixed amount (usually 50) per day
3. A fixed amount (usually 2) per dollar spent

So the best deal can be a function of which car rental company, which FF program, the rate, and the length of the rental.

To complicate matters further, there are a lot of deals out there that give double (or more) miles or give you other benefits. For example, with Thrifty you usually get 250 miles on US per rental. But if you are preferred with US, you will get 350, 500, or 750, depending on level. In addition, you will get 250 HHonors points if you stay at a Hilton while renting from Thrifty. I guess that's the best deal I'm aware of.