true.believer

I have a 2018 Silverado 1500 with 5.3L engine. I originally had a 7.5" lift (leveled front and rear) with 35" diameter / 12.5" width tires on it, with a recalibrated speedometer, that was consistently getting 14.6 mpg. About a month ago I reduced the front end lift to 6" (kept the original rear lift at 7.5" so that the truck resembled more of the factory pitch forward) and installed a smaller set of tires that are 32" diameter and 11.5" width (kept the same wheels). After recalibrating the speedometer I am now consistently getting 13.2 mpg under the exact same driving conditions. Of note is that with both sets of lifts/tires, I used external measurement methods to verify that my speedometer and tripometer were reading correctly after the recalibration. I have also forced a throttle body reset to try to get any of the automated computer fuel controls to reset, but it hasn't made any difference. I was expecting my fuel mileage to at least improve a little bit, but cannot think of any reason why I am seeing a near 1.5 mpg decrease in fuel efficiency with the lower lift and smaller/lighter tires. Can anybody help me figure out what I am missing here?