For GPH, just make note of your engine hours next time you fill up and then divide the gallons from the next fillup by the amount of hours and you'll be good.  Unless you know some distances where you boat, you'll need a GPS to figure out MPG. 

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O'
"Little Mascara 3"
2003 266BR,496 MAG w/Bravo III

Thanks!  I doubt Ill bring the GPS every time we go out.

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2009 185ss- 220hp MPI 4.3, Social Swim, citrine full hull band and most of the toys!