I track my MPG religiously, both on a notebook I keep in the truck and also on the Fuelly app on my phone.
I did not notice any appreciable difference in MPG from the new tires. I did, however, apply an adjustment due to the larger diameter of the tires. So when I calculate mileage, I take the miles shown on the trip odometer and multiply them by 1.024 to account for the larger diameter.
I got the 1.024 number this way: Set the trip odometer to zero, hooked up a GPS with a trip odometer, set that to zero also. Drove about 300 miles. At the end of the 300 miles I took the miles shown on the GPS trip meter (which I figured were my "actual miles driven.") and divided by the miles shown on the truck's trip odometer. 1.024 was the result and now I know if I multiply the trip meter miles x 1.024 I will get the actual miles driven.
Though as far as MPG goes, let's get real here: We're talking about a Suburban, so "MPG" is sort of a joke anyway. Around town MPG is abysmal, because every time I stop and start again I've got to get nearly 3 tons of metal moving. So 11 - 13 MPG is the norm for that.
Then, on the highway, I should point out that the primary purpose of my truck is to be a tow vehicle for a trailer. Our old trailer (a T@B Clamshell) was about 2,000 lbs and our current trailer (An R-Pod 179) is probably 3500lbs loaded.
Compound that with the fact that we live in Denver and usually go "over the hill" (i.e. over an 11,000' pass) to get to wherever we're going and yeah, MPG is pretty much the same 11 - 13 MPG no matter what.
Now, very rarely I will take the Suburban on a long road trip without the trailer (I've actually only done that twice in the 3 years I've owned the 'Burb.) On long road trips without the trailer it gets surprisingly good MPG. I've gotten as high as 18.5 MPG and that was on a trip from Denver to Craig, CO, that required me to go over two high passes (Eisenhower tunnel at 11,000' and Rabbit Ears Pass at about 9600')
