New wheels and new tires, slower?

so i put on a new wheel/tire combo on my base 2.7 truck. i did my research and my new wheel tire combo should be about 3-4 lbs less per wheel than my previous setup. i didnt want to negatively affect gas mileage or performance. i went from stock steel base wheels and p265 70 16 hankooks to scs f5 wheels and p265 75 16 hankooks. i figured with this decrease in rotational weight per wheel, i shouldnt notice a distance at the least...possibly get a little better? well, i find myself having to downshift up hills that i didnt have to downshift to go up before (at the same speeds). am i having this issue now because my tire is a whole .5" bigger in diameter, or is there something else i didnt take into account in my research?