According to my OBDII reader my 08 Base is getting about 17.5

 

I average about 15 city driving and 17 if I am lucky on the Highway if I dont go over 70, but usually if I keep it between 60-65.

 

My 08 3700 gets ~12 city and I've seen 21 highway pretty regular.

Which is about average for most larger/mid sized SUV.

~19 with the wife driving.

Cut out the jack rabbit starts and you'll see better mpg's

I stopped buying for mpg's years ago, if you're looking for mpg's, Hummer is not the vehicle for you.

It's a nearly 5000 lbs. vehicle, unless you're over 400hp or better you won't be breaking any records, so no sense in flogging it.

Even the 300hp LS 5.3l v8 does'nt really have what a 2.5 ton H3 needs to "go fast".

As far as the 3700 goes,, it pulls better from a roll than a dig anyways. Most DOHC engines like revs even with VVT.

 

Oh and, I run no lower than 89 octane, usually run 93 in everything I own.

Your engines PCM will pull timing for lower octane ratings which will hurt performance and mpg's in any obd2 vehicle on a stock tune.

Generally speaking,, GM factory tunes are usually done for emission legality.

If you're really wanting a few more mpg's out of it, (and a little more preformance) have a professional tuner look @ your setup. I mean a guy that is a real professional. Not the neighbor buddy proficient with a hand held or a lap top or tried a chip once.

As a 4 time f-body (Camaro v8-v6 89-01) owner/builder and OBD1-1.5-2 domestic tuner car (J-w-x-body) builder as well as also using stand alone ECM-PCM's I can tell you in general, hand held "tuners" and "chips" are frowned upon.. They are usless when compared to a real tune such as those that can be had from a professional using HPT, or EFI live, tuning software..

Just my bucks worth..XD