I'm going to throw this out there from the other side. I think another variable are the H3's themselves. I drive very conservatively, run stock tire size, mild level, Volant intake, GM dual cat exhaust and PCM tune and I only get 14 mpg around town. In fact, 13.83 when I got gas this morning. On the highway, the best I've ever gotten was 19mpg, usually upper 17's. I've run out of things to try to improve it, so I just live it.
Maybe it's just my H3. I WISH I could get 16-17 in town. From what I read and what I get, it varies quite a bit. I hope you get one these good ones I hear about

 

Guess what we just happen to have here in the Central Texas area? FT. HOOD and nearly 100,000 GI's! Add to that that they come back with a year of tax-free earnings and someone is going to be paying those prices. Even before the production line shut down the dealers were buying up every one they could get for resale and selling them quickly. Until recently when gas started hovering just under $4/gal the H3's wouldn't stay on a lot for long. Now that the production line has closed and the leases are coming back, the prices have climbed 15-20% since last March when I bought mine. Luckily, it doesn't seem to matter in this area whether it's an Adventure or Lux or Base, they all go for the same price around here. Unfortunately it's also higher than in Houston or Dallas for a similar vehicle.

 

I have an 06 H3. My best ever mileage was 21.22. This was on a limited access rd, a rural 2 lane, for about 4hrs at 60-65. Even going through the little jerkwater towns having to stop occaisionally, still over 20. On average I see 17.5-18.2 driving at interstate speeds. We both try to keep it below 75 because you're basically pushing a barn door. The aerodynamics SUCK!! As far as going from a Hemi Dodge to this thing, well you may have to resist the urge to wanna open the door and help it along with your foot!! j/k, but there's no "power" to talk about with an I-5. I can pull 2 snowmobiles, 4 big guys, all of our gear, and still see maybe 13.5-15...depending on how fast we wanna get there!