Gas tank fast use 1997 speedster

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Wanted to see what others had to say on this subject. I know theee boats rip through gas fast but I feel like something might be off with mine.

So I go riding for hour. At full throttle my gas gauge is resting right on the left edge of the half tank mark. At idle same thing. Put on hoist. Still says same thing.

Come back next day and when I drained from the drain plug I noticed either a tiny bit of fuel or oil on top of the water in the lake. Nothing crazy. I assume that was just from the fumes that re liquified.

But when I go to start boat and go ride my gas tank is all of a sudden at the quarter tank notch. Clearly lower than what it was the day before. When I hoisted it the gas showed at that same half tank notch. And then the next day it just magically goes down to quarter tank ? I know the gauges suck on these things. But I have no clue why it would just drop like that.

I have a twin 85 speedster with 27 gal tank.

Thanks for the help.
The inside hull of your boat will have oil in it. SO... any sheen that you see in the water after draining the bilge is normal.

As far as the fuel... these drink a lot of fuel. A pair of 800's will drink 17 gal/hr. SO... you have about 1:30 worth of fuel. (at wide open)

In your comments... you already answered your own question on the gauge... they kind of suck. The sender only has a limited amount of switches, and if your float has a missing magent... the signal sent can shift. Also... these boats sit at different angles when moving, and sitting still in the water. That alone can move the gauge 1/4 tank.

SO... everything you posted seems very normal to me.
Also what was weird though. Is when moving before hoisting it it was showing right at half. Next day took it out. And once moving again it never went back to that half tank mark it stayed put at the 1/4 tank. Which is what confused me
THe sender is in the rear of the tank. SO... it will show high when plowing. (nose high) It will show low sitting still, with someone in the front seats. (and probably on the hoist)

As for the shift.... like I said... there's a float with 2 magnets activating switches. The float can stick a little... or if one magnet is gone (fell out) the resolution of the gauge will drop.

If you are worried about the "True" level... open the deck lid a take a look once in a while. But it's really not that critical. You know that boat is thirsty, and you know your range at full power. Pull it back to 70% and your range will almost double.
The "Textbook" answer is normally around 70% power. with a boat... things change all the time. If you are loaded up... or run with a Bimini top... 60% will be better. Basically... you want to be up on a full plane, but not causing more drag via wind. On a boat like yours... with passengers... probably around 35 mph is best for fuel usage vs range.
Still trying to track down my leak. I fixed a coolant line. Which helps. But I notice I can idol for hour and hardly any water but after awhile or planing I still am getting some. But def not as much.

Any experience changing out the bilge pump. ? So hard to get to
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