I do not know Alamo's policies specifically, but I know first-hand that Hertz does not credit you for additional gas at return, while Budget does.

(This can work to your benefit with Budget in places where the differential between gas prices nearby is high--you can rent an almost-empty car in NYC where gas is $2.75 a gallon, drive across to Jersey to fill up where it's $2.25 a gallon, and return the car full to Budget, where they'll credit you for 15 gallons at $2.75 a gallon. Boom, $7.50 free money!* And maybe more, as I think the credit also takes some taxes off.)

I also know that Enterprise does not credit you for additional gas, and since Alamo is owned by Enterprise, I would guess that you would not receive a credit.

I personally would try to return the car at a half a tank unless you can get confirmation by an Alamo employee (or, even better, a manager) that you will receive a credit.

FWIW, rental companies measure by eighths of a tank and will usually round up if you're between two eighth marks, so you can come out a buck or two ahead by returning the car at 3.5/8 of a tank.

(In fact, I once rented a car from Enterprise that very clearly had 7/8 of a tank but he marked it on the sheet as 6/8, or 3/4...and I then returned it at 5.5/8 and they marked it as coming back with 3/4. )

I do agree that it's a hassle, though, to try to get the gas on the right mark. 99.9% of the time, airport rentals start full and (barring the purchase of the prepaid gas option) are due back full, while returning with partial tanks is much more common at off-site neighborhood locations that don't have on-site fuel pumps. This problem is particularly endemic to Enterprise, which does not really have a systematic procedure in place for filling rental cars up between customers for several reasons including tight staffing, tight fleet utilization and turnaround times, and a very low-cost operation. It would be nice if they would change their procedures to credit people who return cars with full tanks, because one of the most annoying things that they regularly do is rent cars out with 1/4 or even 1/8 of a tank of fuel in them, which is painful to try to handle when filling up and when returning. I suspect they won't, though, unless massive public upset causes them to lose their precious J.D. Power rating over this subject, because they make a lot of free money off of people returning with more gas than they got the car with.

*Excluding tunnel toll, of course, but presuming you had reason to be in NJ anyway...