I rented a car from a major company and the agent walked around the car with me and we looked inside to agree there was no damage. He then started the car up and showed me the gas gauge was indicating full. He said when I returned the car they would fill the tank and charge me or I could fill the tank at a filling station before returning it. I drove out and after no more than one mile the gas gauge needle moved down to the position between the full mark and three-quarters full. I was surprised so I pulled into a filling station and put in three gallons of gas at over $3 per gallon. No big deal but it was obvious the last person had filled the tank until the needle indicated full and stopped. But the tank was not full. When I returned the car I “filled the tank” before turning the car in. The agent again walked around the car and checked the interior. I told him my experience with the “full tank of gas” and said I wasn’t complaining and didn’t want to be compensated. I suggested the company insist on being the only place to gas up so as to make everything equal. The agent all but called me a liar. I again said I didn’t want to be reimbursed for the gas I was just informing him. He still would not believe me.
Well, that is not going to happen. This does not seem like something to really worry about. After all you could have tried to do the same thing with the gas gauge. But if it did not work then you might have spent more than a lousy 9 dollars
Different experience
I rented a new Dodge Journey from a major rental company. They were as nice and friendly as could be. Besides the free model upgrade, unlimited miles, great price, etcetera, the car was very close to 3/4 full on the gas gauge.
It was marked on the agreement as 1/2 full and I was instructed to bring it back on 1/2 or I would be charged for gas.
CSA
Yeah, gas gauges aren’t that accurate. Neither is a fill up. Sometimes it is more or less full than others. Plus the previous renter filled up a few miles away - some airports don’t have gas stations close by - and the rental agency drove the car around a bit before you got it.
It is $9, your comments fell on deaf ears, rental life isn’t perfect Accept it and move on. Or rent from another company next time.
Although it’s possibly nothing to really worry about…
compare that experience to the one I referenced above. The rental business is competitive.
Which place would you return to or recommend to others? Would it be the one @patgurr described above or the one @common_sense_answer related above?
CSA
The OP saw fuel gauge drop and stopped to put in 3 gallons. Who does that unless they are looking for something to complain about. They then tell the person at check in how to run their business as if the check in person can do anything. This is a case of making something more important than it really is.
Gosh. I just wanted to share an experience. Just wanted to do a little communicating. You guys took me to task. In effect, called me cheap, trying to run other’s business, and that I should cheat the next renter. But I forgive you all.
Some of these guys will eat their own kind! 
My comments were supporting what you were relating. I agree with the rental folks in your experience being less than reasonable and not listening to a customer and treating them as untrustworthy and cheating.
The employee that waited on you had a chance to make a good customer or chase away a good customer. They chose the latter.
I am fortunate that I found a rental company not too far from me AND they are located away from am airport, so their prices are more reasonable than the agencies nearby which are located conveniently at airports. They are nice as can be and I’m sure your experience there would have had a different outcome.
I’m very thrifty, but more than that, it’s the principle of the thing! I’ll go where I’m treated decently and taken at my word.
CSA
My guess as to what happened is that the previous renter of the car fills the tank the way I do–As soon as the nozzle clicks off, I hang the hose back on the pump. I have had some.pump nozzles click off prematurely. I am guessing that is what happened in the OP’s case.
I don’t have any real problem with rental companies.The last rental vehicle I had was when my Sienna minivan was in the body shop to repair some.minor.cosmetic damage​ when a another person backed into me in the Walmart parking lot. That person’s insurance company insisted that I have the repair done immediately. I said that I needed the van to transport me and my fellow musicians to a.rehearsal and concert. When I went to the rental agency to pick up the rental minivan, I was asked if a Dodge Caravan would be.o.k. I said that I really wanted a Duke Ellington Caravan. The agent told.me that the Duke Ellington Caravans were.only rented to Sophisticated Ladies. I thought about suing because this violated my civil rights.
(One of the pieces.on our concert was Caravan from Sophisticated Lady. I really like that piece. At least we had a Caravan to drive to that performance).
Topping off the fuel tank when you noticed a possibly malfunctioning gauge was something I would consider smart. What if there was much less fuel in the tank and you ran out. I would have informed the rental agency of the inaccurate gauge. I had a Toyota Corolla rental with plastic interior trim pieces that kept falling off which I ended up storing in the trunk. Of course I informed the rental agency. They apologized and told me a lot of the new Corollas had that problem and Toyota was working on it. You did nothing wrong. I remember long ago businesses having forms and a complaint/suggestion box to drop them in with the purpose of improving customer service. I occasionally experience a rude employee and shrug it off. It sounds like you did the same.
When you fill the little form out when returning the car, they usually ask for the mileage when the car was filled. Also some ask for the sales slip for filling the tank. Yeah a gauge that says full can have 50 miles on it and still say full. I regularly would fill up about 15 miles from the airport where it is convenient. All in all this is no big deal. Not many people are going to pay the $10 per gallon charged at the rental agency. Secondly, I have never had an agent walk around the car with me and explain all the controls. I usually have to sit there for a few minutes figuring that out for myself.
Can’t even go to sesame street without encountering Oscar the grouch, the previous renter was a lucky guy, I doubt the rental company would have a vested interest in making money off of oddly performing gas gauges. 3 gallons if needed might move my gauge from full to past full. I do not know why it happened, but it did.
Like I said, I never asked to be reimbursed, I just wanted the agent to know what I found out. I was curious and that’s why I stopped and topped off the tank. I learned something. I thought the agent would also want to learn something. Heck, I thought some on CarTalk would like to learn something. By not asking to be compensated I would think the agent would have believed me. Why would I lie to him, I had nothing to gain. But he kept saying I was wrong because the tank was full when I drove the car off the lot. He wasn’t there when I first topped off the tank so who would you believe, me or him 
My own fuel gauge experience is that it goes a little higher (towards more full) after driving a couple miles, if it changes at all. So what you are describing is definitely unusual. I’m guessing whoever filled the car before you didn’t quite fill it quite all the way to save a few dollars. It worked out for them b/c of the idiosyncrasies of the gauge. Seems like you could have done the same thing, right?
One time I forgot to the refuel a rental car and got a big surprise. They charged me $9.89 per gallon. I’m totally serious. I asked why so much, the staff guy there looked a little sheepish, but said it was a "convenience fee’.
Years ago when gas pumps were less complicated a very frugal fellow co-worker told me that after the pump stopped he’d always tip the hose to pour the remaining gas into his tank. One day I pulled into the gas station right as he was leaving, and you know I didn’t go to the same pump 
I don’t know what size place you went to but usually the agents are the ones behind the counter. They never see the cars. The ones in the parking lot just check cars in and out, clean them and park them. Just really nothing new to any of them. Like they don’t know the tank might not be plump full? All they want to do is look at the needle and check the box that its at full.
I used the term agent – which to me means he is employed there. Next time I’ll call him a “staffer”. I had a car once, long, long ago, and the gas gauge would show fuller than full when I filled up. I could drive 80 miles and the needle would not move making me think I was getting 80 miles to a gallon. But just after about 80 miles the needle would start moving down and after that it traveled pretty fast. I hated that gauge. My boat gas gauge will show below empty yet I still have a few gallons in the tank. I put marks on the tank so I have a better idea of how much gas I actually have. And I always carry one extra gallon in a container when I hit the water. Why the container you ask – I like to keep the tank low and use fresh gas on each fishing trip. And if I see a boat on the water that is out of gas I can help them. My preference is for a gas gauge that is very, very accurate. My 2016 F-150 counts down how many miles I have left before the tank is empty. But I don’t know if it’s accurate and I sure as heck don’t want to find out. I have always said – there is no excuse for running out of gas in any machine that is used for transportation. That goes for a motor scooter, car, boat, or airplane.