fuel gauge accuracy?

[Milemaker13]

Hello, I have a 78 cobalt trihull. I'm wondering how accurate the fuel gauge may be.
We filled the boat up, gauge said full. Went out, gauge says half tank... All day, start to finish.
Afterward, filled tank back up. Like ten gallons in a thirty gallon tank (I was told, I saw the tank but didn't see tags, ect).
So, what can I expect from a boat fuel gauge? If I replace my old with new, can I expect an accurate, somewhat linear reading? At least at rest in calm seas?

 

[Georgesalmon]

Kinda depends on the tank. If your tank is square or rectangular, same size bottom to top. Not bigger on the bottom or triangularish it can be pretty close. If the boat is setting level and the tank is level. Mine is very close on the trailer, not so much on the water. A new gauge and sender will not help. Just learn your tank. You already know that 1/2 full means you have 20 gallons left.

 

[sasto]

In all honesty I know of only fuel gauge that is consistently accurate. That would be Tank Tender which is pneumatic....and fairly expensive. Can you measure your tank and convert square inches to gallons?

 

[airshot]

When I first purchased my current boat(used) I had some water in the fuel so I drained and cleaned the tank. When I filled it back up I did it in 5 gallon increments and put a tiny mark on the fuel gage at each increment. That was 4 yrs ago and to this day It still reads quite well. When I get fuel I estimate how much it will take just to see how close the marks still are. I am normally within 2-3 gallons of my estimate or closer.

 

[Silvertip]

Float type fuel gauge sending units are fuel "level" indicators -- they are not fuel "quantity" indicators. So the only calibration you need to be interested in is EMPTY. The fuel sender arm can be bent and in some cases the resistance unit itself can be adjusted up or down a bit if needed. Fact is however, attitude of the boat and shape of the tank dictates how accurate or linear the fuel gauge will read. You know when the tank is full because you fill it. Anything in between is sort of a non-issue. But you really want to know when it is empty so adjust the arm/sender for and empty reading when there is (pick your amount) of fuel remaining. That amount may be 2 gallons to 20 gallons depending on how big the tank is. It is also best to make that calibration with the boat in the water so you get an accurate reading at the attitude the boat normally sits.

 

[dingbat]

If you want more consistent readings, swap out the lever type senders for reed switches. More "accurate"...no more crazy level fluctuations

 

[JoLin]

As stated, it really depends on shape and positioning of the tank. My previous boat had a tallish rectangular tank that was mounted crosswise (port to starboard) in the engine room. The fuel sender was mounted dead center side to side and front to back. Once I put in a reed type sender, it was always accurate to within 1/8 tank. On plane, off plane, pitching, rolling, it didn't matter.

My Carver has a 7 foot long rectangular tank, only about 18" deep and 18" wide. It's a belly tank that runs fore-aft the length of the cabin, and the fuel sender is located all the way at the rear of the tank. The only time it's even close is when I'm idling or at rest. Soon as the bow lifts, the fuel runs to the back of the tank and gives me a false high level reading. It's a pain but there isn't anything I can do about it.

My .02