1: If I plan to do alot of driving, I buy pre-paid gas AFTER checking www.gasbuddy.com for local prices and comparing what the agency charges.

2: When I might run out of gas!

3: When I return the car if it looks more than a millimeter under the F point on the gauge.

4: Some rental agencies in some markets will demand receipts to make sure the gas was purchases within 2 miles of the facility..DOLLAR used to do this at DEN. Not sure they still do. I have been asked this at OKC also, but I am sorry I forget which company it was. Enterprise has been known to ask this too.

5: I have only had one situation where the gauge never dropped below full, and I took my chances and did not refill the car. The gauge was either broken or the car was amazingly efficient (toyota yaris). I drove at least 40 miles in it. They CAN tell...they can see how many miles you have driven the car since you took it out, so if you come back with less than 10 miles on the thing (other than wasting your money!) they may ask you to provide proof you filled it. Some cars have gauges that go well over F, and the needle will only get down TO F during your rental. You can take your chances, or just put the 4 gallons in.