9.4 mpg with 37s and Gobi rack - whats wrong

[dna]

I really never tracked or cared about my mpg since I've had my 2003 H2 over the last 4 years. Though I recall when I first got it that I was getting 12.5 mpg, and that was when drivin' it hard (as I always have and continue to do so) - if I drove it like an old fart I'd get almost 14mpg.

Anyway, a few years ago I put on a Gobi rack, and recall that the rack caused quite a change in the rpm to get to the same speed - especially on the highway. Also about a year ago, I installed 37s on the truck.

Now with the higher gas prices, I'm starting to track my mpg. No matter how light or hard I drive it, I'm averaging 9.4mpg. Does this sound right for my setup? Could the larger tires be causing false readings - do I need to compensate for the larger tires??? I know that speedo are affected when you increase the tire size, so am I maybe getting better than 9.4mpg???

Lastly, anyone ever use a Superchips module in their H2? A buddy installed one into his Duramax Dodge truck and claimed 11mpg increase while 100+ hp increase. Hard to believe for a cheap ($500) plug and play, but I'm now seriously thinking of getting one. My bud said the Superchips module will have less effect on a gas engine, but their website still says 20+ hp gain while getting better fuel ecomony.

Thanks