New fuel pump installed mileage way down
 

..lll

If the fuel pump motor itself was replaced, hopefully the past gauge sending unit float position was not altered.
If, however, the entire module was replaced that could account for different fuel tank level readings. Not a major difference, but still noticeable.

Once you become accustomed to the new readings and the miles traveled you will probably adjust to the changes. If your vehicle runs as it always did except for the hard start problem nothing should have changed with it's fuel consumption.

If the different readings are a problem that you have difficulty accepting, you might need to have the repairing location do what is necessary to correct the problem...

The whole pump was replaced as the cost was 800. My issue is with the number of miles driven and how much gas the gauge says I used. I just went out for maybe a 10-mile drive and the fuel gauge went down a 1/4 tank, that's a lot considering the size of the tank on that model. When it gets near empty again I will fill it up with the engine on to make sure the needle goes to full when the gas pump shuts down and then monitor my mileage from there. Hopefully better news with the next tank.

If you already know this fine, If not, not a good idea to continue adding fuel when the pump shuts off ! Creates problems with the Evaporative Emission Control system.

I meant the actual gas pump at the gas station Sorry... Going to fill up with it running and make sure the gauge says full when the gas pump shuts off :)

Would be just as good if you just left the key on instead of the engine running.

Update. I filled the truck again and it was showing empty and the low fuel warning message came on, but it only took 16 gallons of gas. The tank holds 29, so I still had 13 gallons left which is almost half a talk. So what is the cause here? is the float just messed up or what should I do next.

If the installer had a problem installing the replacement fuel pump module the float could have been altered, You might inquire if they can recheck and correct the float level readings. It will require that the fuel tank be lowered, again, the module removed and the float rod readjusted , if that is what they find wrong with it. It will require a meter to be connected to confirm the correct reading at both the full and empty physical positions.

So I have the truck back where it got the new fuel pump put in. They pulled it out don't see anything wrong with it. Now they think it might be the instrument cluster, I do not think that is what it is. New fuel pump does not instantly make the gauge go bad. Does the pump itself have to be calibrated with the float or does it just come in a certain spot and you put it into the gas tank and it moves and starts relaying the info to the gauge? Any more help is appreciated.

The fuel pump module consists of the fuel pump motor and filer sock. It also has a float attached to a rod that moves a wiper along a variable resistor winding. The resistance changes are what the dash unit gets and registers the level You cannot tell very much by just looking at the rod the float is suspended on ,the resistance reading is what must be checked and the fuel gauge watched as the float nears the top of the level in the tank. There is a good possibility the people that replaced the module are not sharp as to how to use an ohmmeter to confirm proper operation. Even then the reading will come close if the gauge is watched as the float reaches the top as with a full tank.
Of course there is always the possibility the unit is defective in which case they should stand good for correcting it.