I am jealous of these numbers. All I have done is the FJ suspension, daystar 2.5/1.5, and 265/70 Nitto Terra Grapplers. I just drove from Kansas City to Silverthorn CO and was getting between 15.5-16mpg calculated. I know this could be a LITTLE bit off because of the small increase in tire size.
But for 100% highway and keeping it around 75mph, thats a low number.
The truck has recently had all fluids changed to synthetic, new air filter, removed carbon filter, new-ish plugs and cleaned MAF.
I have broken 20mpg ONCE on another trip to CO over the summer. However the highway trip was 16-17mpg, and once I got into town I was getting around 20mpg. Its a very windy trip across the flats in KS, but I have been on other long road trips (headed East) with results that arent much better.
I have an 05 with 76k.