Stock tires (285s-33s), flowmaster EXTREME, Levelled, roll-n-lock top.
Started trip with just under 5,000 miles. In 28 days, I logged the following data. Hwy speeds were 75, three tanks were in city (can you guess).
Oh yeah, I LOVE my HUMMER!!!

MilesGallonsMPG251.517.214.6404.523.417.3261.717.515.0361.822.516.1282.817.716.0368.423.815.5335.419.617.1302.619.915.2315.420.815.2301.420.0815.0313.519.316.2397.723.916.6379.922.217.1406.624.216.8240.314.216.9316.319.915.9

5239.8326.1816.1

 

That is not the way it was suppose to look...
5239.8 miles, 326.2 Gallons = 16.1 MPG

 

Well my fuel economy flat out sucks! Three consecutive tanks of 87 octane 3/4 highway at 70-75 mph and 1/4 city driving and I am averaging 12 mpg. Since I haven't recalibrated speedometer for bigger tire size (295/75/16), I estimated only an extra 1 mpg so it looks like 13 mpg overall. I am less than pleased considering it's an I-5. Had I known that, I would have bought the Alpha or a used H2/H2 SUT.

 

I drive an H3T alpha with a SlantBack topper, 275/55 R20 at 39 psi, a Warn XD9000 winch in a DOI bumper.

My commute is 32 miles each way, 11 on the So Cal freeway and 1,000 ft elevation change. The first/last 3 miles are a winding mountain road with a 25 mph limit (you don't need the gas pedal for the first 3 miles in the morning once you get it rolling, only the brakes, but you need lots of gas going home). I get 15 mpg on my commute.

On a trip to Nevada going 9 mph over the limit, I got 14 mpg on the flat and 13.5 going up hill and coming back with 1,000 lbs of rocks and gear 13.75 coming down hill and 13.25 on the flat.

I assume you have checked your tire pressure and air cleaner. Try driving like an old lady or a Prius driver for one tank and see if there is a difference.

 

Base T, I5, leveled, auto, somewhere between bingo and nascar, 50/50 city/highway.

I've seen as high as 20 mpg and as low as 12.66. Overall 1060.91 gallons of the low octane stuff got me 17361 miles. 16.36 mpg average.


WGW

 

I routinly get 17+ to 20+ hwy since I use cruise control and dont go over 2,000rpms. I notice 22mpg at 45mph (I do have a bed cover on my alpha T and with the recomended psi).

City driving is another story, i ve seen as low as 7 but thats also because its NYC driving. Lately I tried using the higher octane stuff and Ive been getting much better city economy just with the 89 rating. For hwy I use the cheap stuff but drtiving stop go, get the 90 or 92 if you can find it; it does cost a good 10% more but it will get you to 2nd gear so much quicker and save u some cash.

 

Two things happen with big tires;

The first, which you touched on; you can expect a drop in MPG as it takes more energy to overcome resistance and the additional inertia from extra weight when accelerating.

The second is that your truck is covering more distance per rotation of the tire than before... so 100 miles might actually be 106 miles. If you use the ODO reading you will be cheating your mileage.