1990 38 convertable fuel economy

[Zultan]

I'm interested in buying a 1990 38 convertable with twin detroit 425hp 671T's in it.Can anyone tell me the expected liters/gal per hour i could expect from this boat and anything else that i should be aware of.
Thanks

[Martuni]

I have a 2002 40 Open with Yanmar 465 HP engines. She burns around 1.5 gallons per mile at 26 knots.