John,
Good job for following up on a negative performance change that showed itself in fuel economy. It is your new tire size.
When I had to drive a rig with less than optimal tire/gear combo, I drove VERY different. Normally with the automatic I wouldn't shift higher than 3rd untill 35 mph but when my tires were a couple sizes larger and the gears were not yet corrected, I made this adjustment, 3rd to 45 and OD was never on unless I was up to highway speed (55 in my area). This wasn't perfect or even ideal but I did realize a couple miles per gallon back.
Another way to compensate for loading the engine is to unload the engine. Remove the crankshaft driven fan/clutch for an electric setup. It is exactly what I did to recover some of the economy originally lost when I "overtired" the 1st time. The crank driven fan...it's the devil in a V6, let alone an I4!!!
. I've even recovered over 2 mpg in a build v8 losing the crank fan for an electric