Fuel gauge hasn't got anything to do with it.

One thing that is important is to fill up from the same pump, at the same station, with the truck pointing the same direction to get an accurate re-fill.  I've seen people claim fluctuations like you're talking about based simply on the fact that the corner with the filler neck was at the low point of the truck on one fill up and on the high point of the truck on the next.

Even more important than the other though, is to find out your odometer error by checking against GPS.  Of course, it's too late to check it for the old tires, but at least you'll know your calculations are right on the new tires.  If you look at the ODO on the dash and the gps, then drive until the GPS is 10.0 miles different, you'll know the percentage error with very little math.  If the GPS reads 10.0 and the car reads 10.4, then you have +4% error and need to subtract 4 miles for every 100 the car odo tells you that you went.  If the car says 9.8, then your error is -2% and you need to ADD two miles for every 100 the car says you went.

And last of all, average at least 5 tanks of driving before you make any claims or get worried about fuel economy.

It's my initial feeling that the tires shouldn't have made that much difference.  But if you were already near some kind of tipping point, such as the tires are just enough bigger to keep the transmission from being able to use overdrive as much as it did before in normal driving, then yeah, 1-1.5 mpg could be observed.

-Dave G.