In general I find the buy a full tank deals to be a great deal for the rental car co. You pay for a

In general I find the buy a full tank deals to be a great deal for the rental car co. You pay for a full tank of gas and can return it empty. Anything left in the tank above vapors is your gift to the rental car co.
You have to do your math to decide whether a full/empty car hertz rent rental or a full/full policy is better. Which, as J62 said, have nothing to do with the airport but are set by the company (or by your type of "deal"/contract).
Sorry, because `i mixed two separate messages in the one post I think I may have confused you. I wanted to explain car hertz rent rental the rental car fuel policy in Spain, but also comment on Alicante's new refurbished airport facilities. Airports have nothing to do with fuel policies.
In a nutshell, if you are hiring for 1 or 2 days and don't intend to travel many kilometres then the FULL OUT and EMPTY RETURN can be VERY expensive, as I was charged 60 for a tank of fuel that purchased outside of the car rental company would have cost 38 and the car hire itself only cost 45.
BUT if your rental car hertz rent rental hire period is say 3 days or more and you intend to travel a greater distance (like Cowboy1968 did) then it makes little difference. I was just saying that this Tipoa Car Hire offered a FULL OUT / FULL return which is the best kind of deal if you can get it..