need help with my 1964 pontiac catalina

I recently bought a 1964 pontiac catalina and the gas gauge is broken. Does anybody know how to fix it, and if its easy to fix? Also, does anybody know how many miles per gallon it should be getting? Its got a 389. So far I've just been using the odometer to help me calculate when i need to fill up next. I believe it has a 20 gallon tank...correct me if Im wrong.

According to the owners manual it is a 25 gal tank. My 64 Bonneville gets between 12-15 MPG.But I had a 61 with the same engine that got about 18 on a trip.
The guage only has 3 components. The sending unit, the dash guage and the wire between them. To test the dash gauge, pull the wire from the sending unit in the tank and ground it(with the key on). The guage should go to full. If it does the guage and wire are good.( if it's not, the only solution is to replace it). The most common problem is a bad ground at the tank. Try running a wire from a good ground to the housing of the sending unit. You will probably have to scrape off 40 years of crud to make good contact. Sometimes just loosening the sending unit from the tank and reseating it is enough to make it start working. If this doesn't work and the guage is good the sending unit in the tank needs to be replaced.
Hope this helps.
Lee

I just read up on the mileage my 1960 gets abt 20 MTG im not real sure abt the gas Gauge sorry i cant help i just got mine so dnt know to much abt them yet!!