With our 40 hp Yanmar when we are motoring at 6 knots we are using the best part of 1 gallon per hour... I'd say that 10 mpg is optimistic for our 38' sailboat. According to Yanmar when I can push it to about 7 knots I'm burning 2 gallons per hour!
We usually motor at about 6 knots... 1 gallon per hour equals 6mpg. At 5 knots we are maybe getting 10 mpg...