I have a 73 180 J model with long range fuel. 79 gal useable. Or so it says.
Can someone tell me why it has outboard and inboard fuel fill caps on each wing?
If I fill it from the outboard will it hold more fuel? Up to this point I have only ever used the inboard filler caps. It’s on wheels now, but I’m putting it on floats for the first time since I have owned it. Will floats vs wheels make a difference in fuel capacity?
Maybe when I have been topping it off with the inboard fillers only, I perhaps have been lying to my fuel monitor when I am telling it I have 79 gal on board.
Things to ponder when flying from the lower 48 to Alaska up the coast. . . .

Use the outboards to fill to maximum capacity, ensure the fuel selector is not in the "both" position and wings are level laterally when refueling. I removed the inboard fillers and installed factory plates as the inboards always wept at the cap when full even with new gaskets.

Typically, fuel is maximized on floats due the reduced angle of the wing (the tank is sitting flatter relative the fill port). I can obtain a higher fuel load on floats than on 31" ABW's. I have LRT in the wings and 26.5 on the belly and require every ounce of fuel so maximizing capacity is my priority when refueling. A properly calibrated fuel flow is a near indispensable tool when maximizing onboard fuel.

TR

Yes, it holds more using the outboards. 6 gallons per side? I'm relying on memory but that's close.

I've been told, unverified, that the inboards were intended for fueling from the step on the fuselage when on floats. You cant reach the outboard filler from the step.

I also know of one early model, non factory floatplane, that had the inboard fillers removed because, supposedly, they had been installed without a legal basis or approval on that particular airplane.. I don't know the specifics.

I wonder if the inboard fillers were a part of the factory float plane options??? Pure speculation on my part.

Mine don't leak or weep. I use the outboards when I want a full load and the inboards if I want to leave a bit of weight off.

On my 1975 185, the outboards only take a little more when you are on wheels. On floats it’s pretty much the same. It is also difficult to fill the outboards on the dock so there is no point in using them on floats.