Scot,

My 2cylinder Universal (18 hp approx) does 6 knots an hour on a little
over 1 Quart. At 4/5 knots, if not bucking a tide or a stiff hedwind,
I'd say you'd need about about 26 quarts or 4 Gals.

What is your motoring range for your boat with the 12 Gal tank? Should
be in your Sps? I'd say that they gave you a 250 mile cruising range
with power

Ole Thom