I lived in Hawaii for 25 years. In fact, I was born and raised in Hawaii, and by no stretch of the imagination would I consider Hawaii to have "highways".... Heck, drive 30 or 40 miles in a straight line on Oahu and you will be in the water...:) Combine that with the traffic, except maybe early Sunday AM and I'll bet your cruise control dash indicator rarely get's to light itself up. Hawaii and Maui relieve those distance limitations, but limited access roads are pretty hard to find in those parts.....
What we mean here in the central continental US by the term "Highway" is a long (> 200 miles or so and possibly up to 400 miles) stretch of limited access divided 4 lane roadway with a speed limit of at least 65 MPH, possibly up to 80 MPH. (If anyone should wonder where a legal 80 MPH speed limit exits, try I-20 in West Texas and I-25 in South Colorado.) This is where you load up the CD changer with 5 or 6 of your favorite CD's, set your cruise control, and drive for 3 or 4 hours at a single unvarying speed. Under those conditions, I have gotten as high as 31 MPG measured from fillup to fillup in my '98 LS400. If your bladder could handle it :), you could easily cruise for over 600 miles on a single tank of gasoline.
Under these, granted, very different driving conditions, the slipperyness of the Lexus body and the effiiciency of the powerplant can yield excellent fuel milage, assuming you use Premium gas, make the right choice of tires (Michelin XVS Energy) and a reasonably high (34 PSI cold) tire pressure.
Vern