I replaced the tank in my 1975 SeaCraft 18' with a new one same the same size and it hold right at 49gal. As far as MPG goes it get about 2mpg average. It has a 1992 even 175. If your #'s are correct your getting one hell of MPG. Mine is also carb 2stroke. Good luck.
[ January 12, 2004, 06:26 PM: Message edited by: Mi Time II ]