I have a 9th gen with a V6. With the original Goodyear assurance fuel max tires, I managed to get 41 mpg on a 150 mile trip that was mostly highway. (I made every effort to drive efficiently to see what mpg I could get on that drive) I did not like the dry or wet traction of those tires. I did like the gas mileage. To me, safety comes first, so I replaced them with Bridgestone Potenza RE980AS Ultra High Performance All Season Tires. Not only are these tires not low-rolling resistance, every comparison test I’ve read shows they consistently rate worst in the fuel mileage category.
with that said, I probably get 2-3 MPG less than I did on average with the OEM’s. To me, better traction and stopping distances in both dry and especially wet conditions are a fair trade off for the extra $50-75 a year I now spend on gas.