When we had 1200 miles on our 2wd 05 RX, we took a trip to our beach cottage. It was April, it was cool in the low 60s high 50s, and the interstate (I-40) is somewhat flat. At 75mph cruise control, we got a 25mpg average. This was with no city driving.
Now that it is hot, and we have 4500 miles on it, we get 23 mpg at an indicated 78mph cruise controled speed. There is more traffic now.
We use 89 plus gas in ours.
I think cooler weather makes for better gas milage.