I'd say some of these figures here are a little optimistic in regards to MPG. The only way to know for sure is to measure fuel flow and GPS speed, and correlate the data. Lowrance makes the LMF-200 and LMF-400 gauges that can do this when used with their GP-60R fuel flow meter and a GPS antenna.

I recently tested a 22' Stoker with a stock 502/500HP and it got a best of 2.8mpg @ 2750rpm (I believe the speed was about 40mph with a 26p prop). With a little EFI tuning, we were able to bring that up to 3.4mpg at that same spot (leaned it out from 11.8:1 to 13.0:1 air/fuel ratio).

In comparison, my 30' Scarab with twin V8 outboards gets a whopping 1mpg. LOL