Rental companies have different fuel policies. The most honest policy is: you pick-up the car with a full tank and you also return the car with a full tank. Be sure the tank is really full when you return the car. If not, the rental company will fill the tank for you and their rate for gasoline is much higher than what you pay elsewere.
Other fuel policies are more annoying, like full-empty. You get the car with a full tank (you pay their rate for that tank and that rate can be outrageous!) and you return it with an empty tank. Of course it's never really empty and when you rent a car for a day it's maybe not even half empty. Rental companies never pay you for the fuel you didn't use.
Some rental companies have a half-half policy: you get the car with the tank half full and you return it with the tank half full. Of course this leaves more room for discussion than full-full.
A bad fuel policy can make renting a car more expensive than the advertised price suggests, so check the fuel policy before you make a reservation. All brokers show this information for each car.