Strange fuel tank issue
I have a sonic 35ss, 725's super charged, bravos. The boat has 2 70 gal aluminum tanks which were replaced in 1997. I have only owned the boat for 2 years and bought it as a complete project.
The port side tank uses much more fuel than the starboard . Each engine runs independent on its own tank. There is no fuel valves or crossover that I can see. In fact it is a very simple fuel system . 1/2" line with AN fittings straight from tank to mech fuel pump then to fuel filter and then to carb. Port tank drains 1/3 faster than other.
Here is a list of the tests I have done so far:
Filled both tanks to the top, ran the boat then topped both tanks again, results 45 gal used port . 30 gal used startboard.
I did that test several times with always the save results.

Next I swapped the fuel lines so the port engine now draws from the starboard tank. The problem does not follow the engine it stays with the port tank. Again I filled both tanks to the top, ran the boat for a while and topped the tanks both off again . Port tank took 1/3 more fuel .

There is NO smell of gas at any time and my bilge has no signs of fuel.

Any ideas??????

Do your fuel vents have a loop in them? I have seen them siphon over the side. Loop is important.

Are the engines fuel injected?
If so, are the return lines going to one tank

If carbs, do the fuel pumps have a return lines going to one tank?

Not fuel injected as I said simple fuel system one single line per tank
I would think it I was loosing fuel through the vent that I would smell it. The fuel fills and vents are right next to the helm

Nice comparison. My cat is somewhat the same.
I have always burned more gas Port side. I have ran out of gas port side several times too

i have not done actual measurements like you, but I will always put more gas into the port tank!
I have lead it to be the fact that port drive, at least on my Cat, is turning out thus working a little harder?
I have been told by many that the port side turning out will work harder. As well my port side has always turned slower with the sticks equal,--rpm that is. I always have to give more stick for port to keep the rpms equal.
Not saying this is your answer, due to higher amounts of gas used, because I cannot prove that.
Just saying mine is set up the same, except injected 502's. One line to each motor, and both return lines to the corresponding tanks.

Edit, I have never swapped lines to opposing tanks but that is a very interesting thought and thinking I will keep that on the thought to do sometime.