I didn't check it before but I have an 09 alpha and I averaged 14 mpg 60% city and 40% highway and a mile or two of 4 high and low. Right before I got that number I had a check engine light on and by the code thought it was a o2 but they had to replace the mass air flow sensor they told me... I'm not sure how different the mass air's are between the I5 and V8... Mass air would hurt you and mine only has 20k on it and it needed to be replaced...

 

Just took our 08 lux. On a 2400 mile road trip to visit the rat in Orlando I run about 10mph over posted limits, and managed 15.4 mpg overall. Ordering a cold air intake, electric fan kit, pcm4less tune. And changing out to full synthetic fluids. Next trip to see the rat is over Xmas, I will report back any gains.
I was very surprised as my wife's last truck was a 06 Colorado crew cab z-71, with an intake, throttle spacer and jet chip and it got over 21 highway. I know the h3 weighs a lot more and the awd, but surprised at the 25% decrease.

 

Takes far too long to recoup any investment you make strictly to increase mpg.

 

i have 2006 H3 lux. and adv. over 120,000 miles i average about 16.9 mpg constantly. mixed driving.

 

If you REALLY want to up your mpg's you can remove the front driveshaft, and L/R axle shafts and just go around in 4Hi Lock all the time. Should get you back around 25% more...


But WHY!!!

 

I was going to pull the front shaft before the trip, but I was concerned about any issues that might arise. This spring I am going to remove it and see what happens.

 

Not sure where the OP is, but here in UT winter is starting to set in. For me this always means ~3+ mpg less. Noticed my last couple tanks were around 14mpg, where the same driving in the summer is around 17mpg. I'm sure it's a combination of the effects of colder temps and winter gas blends. How much of the difference is because of the gas I don't know, some claim as little as 1.5%, others say 20% (which is right at that 3mpg loss I'm seeing).

Few possible things to look into. Tune up, new fluids (synthetic), new air filter, clean throttle body, etc. - all things others have mentioned. You can also go with a PCM4Less tune. Better mpg, power, shifting.. Not really going to pay for itself with mpg improvements, but hey, supposedly it makes a big drive-ability improvement. I plan to get a pcm4less when I get new tires, I want the tune to make up for the losses of going to 35s and to correct the speedo.