MPG on a 2005 268 SS with 6.2L Mercruiser

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Does anyone know the MPG or Range of a 2005 268SS with a 6.2L Mercruiser Bravo III? Also any idea on ideal fuel consumption RPM or MPH?

 

We are on the Mississippi River near St. Louis and want take a trip up to Chicago via the Illinois River (around 328 miles) and while there are several marinas along the way, there are a few lengthy dead spots with no fuel access.