I was told as a general rule for EFI engines, divide the HP by 12-12.5 for gallons per hour at WOT. Carbed engines are not quite as efficient so divide the HP by 11.5-12.

So a single 500 EFI with 470/12 = 39.1 gph at WOT

and

Two 500 EFI with 470x2/12 = 78.33 gph at WOT

...just asking but would two engines be more efficient, therefore burning less gph?

...and is there a way to figure mpg at cruise?

Dude really?? C'mon w/ all these questions... Your basicslly asking the same things over & over!!! Your that worried about efficiency your looking into the wrong hobby.

454/502 blocks, N/A....figure 1 mpg with twins, 2 mpg with a single. It should be a little better but for budgeting that will be close....and I agree, gas will be the cheapest thing you spend on a boat. If those numbers scare you, look for one with a sail on top.

Gas, insurance will cost as much maintenance or vice versa as t500 said. And I mean maintenance that keeps it in top shape (no failures when taking it out) My numbers are .8-1.7 mpg twin 454 mags. About $400 a day in fuel. There is a passion for these boats that negates cost.

I would be more concerned about coming up with an average GPH figure. Or, GPH at certain RPMs.

As far as MPG, that depends on the hull. I could squeeze around 4 MPG @ 55 MPH from my last boat w/496.

For my first poker run in the Spectre, I had my friend with me that's a CPA. At lunch he mentioned that before the day was over we would have spent $500 on gas and oil. I told him that was the cheapest thing that could happen to us that day.

I like to be 100% educated on ALL my purchases, I want to make sure I get 100% accurate information, the more people that chime in, the more accurate the info will be.

If the checkbook was unlimited I wouldn't be asking these questions and just write a check for a few million...

I run twin 540ci n/a motors,I would say at 50mph I burn about 15 to 20 gal per motor about 3400-3600 rpm as said before gas is the cheapest part of these boats my 2 cents

YOu can make the tank last a month or you can burn it in a DAY1 Its your choice on how much you use it.
GPH come on. The conditions vary way to much and the throttle position is what matters the most.

It varies by boat. I get better mileage with my 32 AT with twin 502mpi's than I did with my 28 Pantera with a hopped up 525SC.
That is at cruise speeds only. WOT, the AT uses more fuel.