What engine do you have? Chuck mentions the Universal 25, but my Allmand came with the Universal 5416. Here's what I posted in another thread:
Rock Hall to Reedy Island: Approximately 53 nautical miles in 10.5 hours. That gave us an average speed of 5.04 knots. Converting that to statute miles, that’s 61 miles at an average of 5.8 MPH.
Reedy Island to Cape May: Approximately 49 nautical miles in about 9 hours. That gave us an average speed of 5.4 knots. Converting that to statute miles, that’s 56.39 miles at an average of 6.25 MPH
Cape May to Forked River: Approximately 76 nautical miles in about 13.5 hours. That gave us an average speed of 5.63 knots. Converting that to statute miles, that’s 87.45 miles at an average of 6.47 MPH.
What I find especially amazing is that on the last day, we were fighting the outgoing tide for the first two to three hours of our trip from Atlantic City to Forked River. You could easily see the current eddying around the pilings. Yet we still managed to make that our fastest day.
I topped off the fuel tank before we left the first day. When we got to Cape May at the end of the 2nd day, we topped off again. It took approx. 15 gallons to top us off that time. I haven't purchased any more fuel since then. We motored Day 1, motored with a little motor-sailing on Day 2, and motor-sailed almost entirely during Day 3. We had the throttle at full for most of the trip after Day 1, and even during Day 1 after about the first half of the day we were at or near full throttle.
So, we used 15 gallons over approximately 19.5 hours, and covered about 102 nautical miles (117 statute miles). That works out to 0.77 gallons per hour or about 6.8 nautical miles per gallon, or 7.8 MPG.