Fuel tank capacity on 1981 Vette and reserve ?

I put a new Dorman 575 - 080 tank in and it's supposed to be 23.7 gal. tank. I know the original Chevrolet tank was supposed to be a 24 or 25 gallon. After I put the new Dorman in I only put about 4.5 to 5 gallon in and it never registered on the gauge at all. I need to have some exhaust work done on Wednesday so I didn't fill up at the filling station. I did put another 10 gallon in with plastic cans now the gauge registers just a little under 1/2 tank. That's kind of telling me that we have a 5 gallon reserve when the gauge says mt. Is my thinking correct or am I missing something ?

If yours doesn't have a bladder in it then it's not going to register for the first few gallons. Effectively I guess that gives you a reserve capacity. My '73 tank capacity is only 18 gallons so I don't think I have a reserve.

by bladder, if you mean the liner like the original tank had ? no it has no bladder or liner in this tank.

Somewhere in the mid to late seventies, Chevrolet added a safety bladder in the gas tank. Not sure what it looks like so I can't confirm what your old one had. The lack of a bladder in your new one may explain how they got larger capacity assuming that the bladder didn't conform to the entire volume of the tank.