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Does the published fuel tank capacity number include filling the neck? I believe mine is a 38gal. But even when it is towards the bottom of the "E" on the fuel gauage I can only get about 32-34 gallons in it before the pump shuts off. I believe the neck will hold about 3 gallons (from previous neck fills). So I'm thinking that published number must include filling the neck.
If you trickle it slowly, you'll get a whole lot more in there. I have a SB which has a tank rated at 29 gallons. I can easily get more than that in there when I fill up if I take it up to the neck. When I dropped my tank for the Hutch/Harpoon, I put in about 5 gallons out of the geri cans, then about 29 gallons at the pump (this after just about completely draining the tank). That tells me that my "29" gallon tank holds closer to 34. Yours is probably over 40...
BTW, the Harpoon mod is for being able to fill to the neck MUCH faster. The last time I did it, I filled it until it clicked off, and it took less than a minute to get it up to the neck (less than a gallon). Before the Harpoon, I'd stand there for 10 minutes trickling in 5-6 more gallons to get it full.