Interesting. My research went another way. I quoted a 4-day weekend with and without the free-tank PC. With the PC added $30. So my take on this is that the gas isn't free, as the PC adds $30 to the total rental. I didn't check to see if it changed for each car size, such as did it add X-amount for a compact, and 2x for a full-sized SUV ("free" increases with vehicle tank size?). All it does is make it easier to pre-pay for the tank.
I figure they're going to make a fortune on this one. It's real easy to return a car with a totally full tank... just hit the closest gas station to the rental return, and fill to F. It's real difficult to figure out how to return the car with a totally emtpy tank, or at least the tank on E. So people are going to return the car with a half-tank, and loose half their value. Then again, some might like removing the stress of having to return the car with a full-tank (especially as the moment of return can be stressful enough, especially if late to a flight), so that might be worth $30 to some, even if they bring it back with a 1/4 tank.
Steve B.