I have a 40 Albin and a single Lehman 120. I burn "about" 1.5/1.75 gal per hour (as near as I can calculate) at MY cruise speed which is 1700/1750 rpm and 6.5 knots.
I think my boat is a little slow compared to others, but I am loading the engine properly as I can just get to 2500 rpm at WOT.
1 gal per hour seems low to me as well from others I have talked to. They are usually at 2 gal per hour.
I MIGHT buy 1 gal per hour at cruise if you are willing to go that slow...especially if you have a foul current....but 1.5 g/h wide open on a 120? Nope.....