gas gauge reading when full but goes down quickly

My rally fuel gauge works like new. Some number of other owners may or may not chime in to say theirs does too. But then after it is renewed it is supposed to work like new. Replacing a mechanical limiter with a solid state regulator does not renew or calibrate the gauges. A regulator than can never, even during initial warm up period, send more than 5 volts through the gauges helps extend the life and long term accuracy of the gauges.
The mechanical aspect of a gauge is what so many ignore or don't comprehend. Once the heat is not properly transferred to the metal or the metal no longer responds properly to the heat, you can calibrate, bend little thingys, alter voltages, line resistance, whatever. It's still not going to work right.

I like to think most of yall can read between the lines and start to understand why so many TEMP' gauges still work perfectly. By the way... Most oil and temp senders are aftermarket reproductions too.