I know this is an old thread, but I have a 34 Pursuit ( same hull as the 35 Tiara) with the 370 Cummins. It's a great boat and for Sale on Yachtworld. I burn about 22 gal per hour at 24 knots. I would not even consider this boat with Gas engines.
I am still on a quest for a newer Open (3500, 3600 or 3800) and did find fuel consumption test data for all three either from Tiara or BoatTest.com. My old boat (1992 3100 with Mercruiser 454 carbureted motors got 0.9 mpg while cruising at 3300 rpm (25mph). For me, the fuel cost is an issue but also range is important.
Thanks to all for the views and responses.
Tod Kirkland Wa.
1992 3100 Open - Twin Merc Cruiser 454
2005 3600 Open - Twin Cummins QSB 380