Just to clarify one thing. All the rental companies that offer
buying a full tank of gas
upfront that I have dealt with offer this at a price that is either
average or sometimes
better than you can get at gas stations near the airport. If you
choose not to take
the offer and return the car partly empty, then they charge a much
higher rate for
gas.

I always take the tank of gas upfront option if I'm pretty sure I'll
be driving enough to use up
close to a tank or more of gas. Then, you just bring it back close
to empty, don't have
to refuel at the end, and got a good price for the gas. If I'm only
going to drive less, then
I refuel before the airport.