I'm only getting 160 to the tank. But of course I've had non stop problems with this stupid 06 H3. I loved it in beginning but now so frustrated I hate it. My check engine light came on couple weeks ago for catalytic converter bank 1 code P0420. It's been rattling like crazy at idle and thru drive thru's. I just bought new cat for under the h3 from the dealership for almost 800 bucks w only one year warrentee. Seems lame. I removed all bolts in less then 30 min and broke 2 of 4 bolts I've removed and last one above pipe under engine is stripping with no angle to get last bolt so waiting to take it in for that one bolt removed. I'm hoping this will end my nightmare and I can get rid of this money bucket. People say "well don't take it wheeling". Lol ugh that what the car is made for and what I bought it for. I'm gonna try to get a new H3 or just get an FJ cruiser where dealership will honor warrantee. I'm so sick of this hummer I don't even wanna deal w it anymore. DO NOT BUY THE 2006 H3. Waste of money. Constant problems and expensive as hell parts. I strongly suggest fix it and sell it while u still can get money for it and get newer H3. I can't wait to get ride of my H3. So fed up.

 

Jfwiw, you cant calculate mileage by the tank or by the fillup. You gotta use miles driven and gallons used.
400 miles,20 gal=20 mpg. 200 miles,20 gal=10 mpg.
your 160 miles /tank= 160/20 gal=8mpg figuring a fillup on warning, 20 gal.

otherwise,sorry you got the worst h3 in the world.

 

You have named one problem besides the rattle, which may be as simple as a heat sheild, which ours used to do. Free fix DIY. Lots of threads discussing what could be causing your mileage problems. You are going to have issues with any vehicle you buy. It comes with the territory. Get it fixed and enjoy your vehicle.