Power w larger tires

Took a nice run last few days up to E TX and have an observation. Seems a major difference between the prior set up w the RR lift and the new S/L set up is power running up and down the hills. The engine kicks down much more often now. I am thinking its' mostly due to the larger, heavier tires and the higher profile of the vehicle. I never had a problem maintaining speed, but the engine did kick down a gear cruising at 67 mph and cresting an overpass. Wondering if anyone else w a S/L noticed this??? Yesterday was quite windy here so that may not be very representative, but even heading out Tuesday night was similar. I was able to get around 17.3 mpg on the way there, but 15.8 mpg on the way home driving same route.

I don't know what an S/L is, but I do know that lifting the rig and/or getting rid of the forward rake, as well as larger tires makes the mileage go to crap. I compounded it when I added the roof rack. When I first bought the XK, I was getting 19.5 MPG. Now? It's doing good to get 15 highway. So it goes.

S/L = Superlift (4"). Yeah - I was pretty happy with the 17+ mileage going there.