1969 C10 Truck Gas guage, carb issue

[vweater2001]

Okay so on Friday night I changed out my Holley 750 carb for the new Holley demon 625 carb. Super easy install and seemed to make the truck wake up from the previous carb it had which in my opinion was too much carb or just wasn't set up right.

Anyway, the truck runs really good now with the new carb...Friday night I go fill up half tank to just test out the truck with the new carb. The truck had been sitting a couple months without starting because the previous 750 needed to be rebuilt so I went with new/refurbished one from Holley. Changed the carb and it ran quick without little to almost no fine tuning. I backed out the airmixture 2 1/4 turns as that is where the truck seemed the happiest.

After getting the truck to run, now the gas gauge seems to not be reading correctly. 1/2 tank literally turned to less than 1/4 of a tank for less than 20mile drive...1/2 tank usually lasts about 4 days for me for 20mile drives. I think the truck should be giving me better gas mileage for a newer carb, wouldn't it. Should I have to recheck the timing once again to make sure it's running at it's optimal condition?

Can't understand for the life of me why the gas gauge or the mpg's have gone haywire on me...any ideas, thoughts??