1999 ford f150 gas gauge reads 1/2 tank all the time. why?

If it has a new tank and the gauge has not been reading properly since it was installed and wont start now it might be out of gas.

you have 2 options
1--- put the fuel return line into the tank you are using
2--- install a balance line between tank 1 and tank 2 so that the fuel back to tank1 is allowed to balance back into tank 2

properly set up the tanks should be connected so that the fuel gauge reads the total fuel in the tanks and there is a need for only one fuel pump

indicates a problem in the harness to the rear harness
cut wires , incorrect additions to harness
possibly along the chassis or under body

You probably have a ground wire not making contact to ground on the front tank sending unit. OR the sending unit itself inside the tank is faulty. Either way, when a gas guage goes to full all the time (empty or full), this indicates that NO ground is available for the sending unit.

replace front fuel pump because gauge is made into fuel pump and not serviceable. Or poor boy way of getting by would be fill front tank with gas reset mileage drive normal for a day. refill again with gas note gallons used and mileage driven for average miles per gallon. front tank should be around 17 gallons multiply by mile per gallon you figured

You may have run out of gas in the front tank, but the gauge may have been reading the rear tank or the front sender is stuck. Is the fuel line switched electrically or manually controlled? How are the gauge connections switched?

Check the harness connector at the fuel tank for a short to ground and test to see if the tank is properly grounded.

The sending unit in your tank is bad replace it.

The switch that you use to change tanks is probably faulty - the switch supplies power for the sending unit in the tank, and to the transfer switch for the gas lines - use a meter to check it

When replacing the fuel tank often mechanics dont check the gauge, it can be damaged when reinstalling it or if it came with the sendor in it already, often they get knocked around in shipping making them read wrong. And in the shop we always check the oil pressure with a live gauge . you can get one at a local parts store pretty cheap, it comes with a new sending unit and all, and the sending unit is the ground, the gauge has a continuous hot and the sending unit applies a amount of ground to allow the needle to climb.