I too recommend looking at the gas guage at speed. It takes a long time to go from full to 3/4. I suspect 3/4 is more like 3/8s. I've let my 29' (130 gallon tank) get low only once. It showed empty running and went up to about 3/8s at rest. I think your guess of 40 gallons might be close. I would say 25-35. If you are going to play "gas roulette" I would suggest packing at least 20 extra gallons onboard. I always wonder who fetched Reggie when he speed tested all of the Fountains for their performance report with "ten gallons of fuel" onboard!

Update: filled up yesterday and it took 72 gallons. I'm sure with the boat on the trailer you never actually have truely "full". And I'm sure there is an amount of gas in the very bottom of the V of the tank that the sender can't pick up. 72 x 6.6 = 475 pounds of fuel!