Late answer .
Use LPG . Tribeca is a heavy SUV + AWD and Subaru engines like FUEL !!
I use BRC system (Genius max+yellow injectors) 11 years now, but the consuption steel remains about 15l/100 in motorways, in the city you can see even 20l/100 !!
But i loved it, never betrayd me all this years in snow, hunting in the forest of my beloved Poland or cruising in the motorways in the long trip from Greece always heavy loaded !!
My solution, in Greece in short distances i use metro or walk, in Poland i use my wife's car !!