Gas gauge not reliable gauge shows half full one minute and then it process to full without adding fuel.i have not attempted any type of repair

Calibrating a fuel gauge used to be a regular job in the repair workshop but since CAD and CAM became features in car factories it is rarely required.

One problem is the type of gauge used, with moving coil and moving iron types being largely superceded by a hot wire type.
A feature of this type of gauge, or more strictly indicator and not gauge, is during cold weather for instance, the indicated fuel volume after driving 20 miles or so and the cabin has thoroughly warmed, can be considerably more than when starting...

Calibrating the fuel level gauge, indicator (approximator) requires putting enough fuel in an empty tank for it to be exactly half full and then to remove the fuel gauge sender, transmitter or sensor and bend the float arm slightly, refit and repeat until the gauge shows half full...

My friend on your attempt to renew the fuel pump which actually is connected with the fuel gauge you either damage the leveling gauge either by bending it or scratched it to a place where the gauge shows half tank. It is better to remove again and very slowly and carefully check for any slight faults.
Good Luck.

If your fuel gauge is showing you in accurate readings this is normal to all vehicles that are not used to fill-up gas full regularly but sometimes the cause of the inacuracy is due to faulty float valve in the gas tank you need to replace it

If the trouble with the gauge started after you worked on it then you likely damaged or created a restriction to the movement of the sending unit. You will have to remove the tank and double-check your work.

You will need to replace the fuel tank level sending unit.

The fuel sending unit has a rhetrostat its a variable resitor ..As fuel is added the fuel lever mover along the rhetrostat and gives resitor signal to the fuel guage, the fuel gauge interpets this as level indicators to show on the gas tank.

Hello. You may have a problem with the fuel sending mechanism in your gas tank or you may have a fungus in your gas tank which is causing it to show much more fuel than you actually have. Drive to an AAMCO repair shop where they will scan your car's computer for free [some auto parts stores now offer this free as well so call around]. If it is mechanical it should show up on the scan, if not you probably have the gas fungus. Some auto parts stores and dealerships now sell an expensive gas tank additive that will kill that fungus. Regards, Joe

fuel tank vent could be blocked by mud/pebbles etc, not letting the gas fumes escape while your adding fuel making it overflow, check underneath the car near the fuel tank .. something to start with..

probably you got a problem with your fuel gauge sender that goes inside in the fuel tank, that sender is only a coil that works together with a fuel lever,if you want to save some money buy it separate from the fuel pump.

Change the fuel gage sender unit in tank.its faulty.