My 1986 30 Scarab Panther gets 1-1.2 mpg,

A boat being filled in the water vs on trailer can take different amounts of fuel.
My boat will take on more fuel in the water(maybe 5 gallons or so per 80 gallon tank than when filled on land.

BTW, on calm water/light chop, I can get 1.5-1.7 mpg. Rough water will be closer to 1.2 mpg with twin 502mpi's in an 8k boat.

No way at 2.4mpg on Twin deep V. Last 4th I did just under 180 miles cursing the river with pull here or there. About 1.2 smiles pr mile. 502 mpi's, 33 Outlaw

Bingo. I bet that's the issue. This 300 sits a lot more bow high in the water than on the trailer and the fuel filler enters the tank at the front. It must have an air pocket at the rear of the tank. I'm going to take it back down to the store and jack the front of the trailer high in the air and see if it takes more gas. I'll report back later! lol

Be careful doing that. I filled my boat on land and then put it in the garage which has a slight forward slope. Ambient temp warmed up some and fuel started dripping out the vent tube.

Well it took right at 10 more gallons. I redid my math, it took 21.2 gallons the other day. (wasn't accounting for super being 3.02 here) so 31.2 gallons consumed on what I thought was about 65 miles, then I went on google maps and plotted the trip, it was actually 82 miles on 31.2 gallons of gas, for an awesome 2.6 mpg, which didn't happen. Somebody must have put some gas in my boat. It's a mystery.