Electric fuel pump draw

"The thing is" there may be enough load on the circuit that the volt gauge is hooked to, that you are pulling that point down that much.

So first question is "to what is the voltmeter connected?"

And, it would determine what all is on that line. EG if it's coming off the ignition switch, you at least have the ignition system, and the alternator field / voltage regulator, and maybe some other things

Is the fuel pump running through a relay?

If you know the brand / model of the pump, the manufacturer's website "might" show the amp draw of the pump