Is there a rule of thumb or ???? on how much fuel an outboard uses per mile??? I have never seen any kind of guide or reference on it, I am more curious about 90 HP and above, thanks

Most outboards are measured gallons per hour. I have tried to check my merc.200 outboard miles per gallon and i'm guessing it gets about 3 mpg.I read somewhere that it burns 20 gallons per hour at wide open throttle

I run a modified Johnson 3.0 looper that started life off as a 200 and now dino's 308 hp from 6,200 to 6,500 rpm.  On a 18.5 ft Stratos, I can cruise at 4,500 - 4,600 rpm, approx  47 mph and it burns almost dead on 8.5 gallons per hour.   I've never checked it at WOT.  The only reason I know the 4,500 rpm burn rate is I run them for two hours at that rpm breaking them in.  I can say, at WOT, is will go through 40 gallons of gas VERY FAST.  But,  when you're running 80 mph you don't have to run that long to get there.  

Even though this motor is extensively modified, it is ported, jetted and tuned to peak performance at those rpms and with 150 lbs compression, it will burn actually burn less that your average out of the box stock motor.

3 - 5 gallons per hour is your norm.  With most motors 4,500 - 4,600 is where they get max fuel economy.  Based on the motor, the hull design, weight and setup.

Drr,

Like trucks, boats and motors get worse gas milage the bigger they get and the more load you put in them...so theres no steadfast rule.

 But Gambler's about right, on the average, any boat with a stock motor rated hp max. on the plate, will get around 3 miles per gal on and off WOT.

 The Opti's and Ho's get 30% better if you can stay off WOT, if you can't they don't do much better and you'll lose a little top end mph.

  In a big water tourney I've blown 50gal of gas in a day. And forget buying small and modifying it...against the rules.

Check boattest.com   I've never seen a peak fuel efficiency at 4500RPM on any test they've reported.  Most big engines (200-250HP) have a max fuel efficiency at about 2500-3000 RPM with a peak of about 5-6 MPG.

Just what I'm reading from the experts, that's all.

I had a mechanic from the local Ranger dealer tell me that it is broken down like this:

115 hp = 11.5 gallons per hour wide open

150 hp = 15 gallons per hour wide open

175 hp = 17.5 gallons per hour wide open

200 hp = 20 gallons per hour wide open

225 hp = 22.5 gallons per hour wide open

and so on......and from what I can tell,these figures seem fairly close.