Seeing as the other topic is now closed, here is my answer, and this by the way is from personal experiance so i know it works.

Firstly when you go to the Dollar desk with your voucher, ask for the option to 'bring back full'. You can do this even if your voucher states you must purchase a tank of fuel, as mine did from this very site.

When you ask the clerk to bring back full, one of two things will happen: -
If they are having a good day they wont pre-charge you for fuel, this is good. If this happens just make sure that when you return the car that you have topped it up, there are gas stations near to both airports. When the car checker takes the readings etc they will issue you with a receipt which will indicate no charge.

If they are having a bad day, they will still charge you for a tank of petrol, no problem - just a little more work needed. Bring the car back full and when the car checker checks the car, tell them you requested 'Bring back full' they will issue you with a receipt, make sure they write on it 'back full' or some other comment. Then return to the desks and ask for a refund of the tank of fuel showing your original rental and the receipt that you just got.

Despite what anyone might say this works, i have done it and more than once both ways.

The benefit is you don't have to figure out how much fuel you have left towards the end of your stay, just keep the tank as full as you want and make sure to top up before the drop off. You also don't get ripped off from the car rental prices and you don't give them any free gas which is left in your tank.