I've loved my H3, barely gave me any problems for 8.5 years. (230K+) Now, boom! Everything. In the past little while I've replaced: evap canister, radiator, transmission, exhaust manifold/converter, mass air flow sensor.
I thought some of this might help with the fuel mileage, but nooo. In the past year, I'm getting 11 miles to the gallon in city driving. Not much better on the highway. At least 20% worse than it ever was. It did seem to drop suddenly after doing replacing the canister. Checked it and it's not flooded with fuel.
We hooked it to the scanner and everything reads fine. (Trim, bla bla)


Anybody? Anything? I have on the stock size tires. No Thule racks. No fat kids in the back.

 

No mention of cleaning the Throttle Body and MAF? Use TB cleaner on the TB and only MAF cleaner on the MAF. When they are good and clean, make sure your air filter is as well.

Your TB should be cleaned about every 30K miles.

 

You said you checked it on the scan tool. Did you check your engine temperature while going down the road? A thermostat that is opening too soon will result in poor mileage. It might be fine a idle, but take it for drive and the temperature drops. Also try some sort of combustion chamber cleaner. A clean engine will give better performance. A lazy Oxygen sensor will also affect mileage. It will be reading fine, but the values are slow to respond to the changes in the air/fuel mixture. Fuel mileage can be affected by many things. Hope this helps.