4x4's are heavier and generally have larger, wider tires and sit a little higher (for hwy mpg), so it's to be expected they will get less mileage, even in 2wd...Small price to pay for when you need it, or use it. Also, many people put bumpers, sliders and other heavy things plus the gear onto their trucks so you'll get worse mileage too...Add to that the highway gears for the city we have (3.36 for auto and 3.69 for manual, although a little more suited), plus they are essentially bricks in the wind and no wonder these don't get great mpg (they do weigh ~4500 lbs!). But, really, they don't get bad mileage either...they are trucks after all and have decent power (for a 6 cyl).
As for the older models (pre-2009), I ran across an interesting thread on an xTerra forum, iirc, where they were putting the DT LT's on their 2009 I think it was and only gained 5 hp on stock ecu, where the 2008 claimed to gain 26 hp on stock ecu...I'm sure the latter was exaggerated, but the point is that they were talking about how the 2009+ ECU's don't "learn" the mods as well as the older one...So, this leads me to believe that they recommended 91 octane on the older ones, since the tune was able to compensate for the advance timing that the 91 would allow (vs. 87 where it would retard it to keep it from detonating)...Now, why the hp is rated the same on the newer ones using 87, I don't know, other than maybe they changed the tune somehow else to make this up, just guessing...Is the compression ratio, cams and everything else identical between say a 2008 and 2009, or 2010, etc.? Or, I suppose it's even possible they realized that they didn't really *need* 91 octane afterall...
With that said, say it is true, then 91 would be a waste of $$ if the ECU doesn't allow any extra timing, etc. from 87, or if the 87 is good enough antiknock quality to prevent detonation...That is really the only reason you go up in octane. Higher compression (N/A or forced induction being compressed air forced into the cylinders) needs more anti-knock properties, apples to apples, but we also have aluminum heads/block and ECU tune (vs. a carb and non-ECU), so 9.8:1 is not really that high like it once was. It is getting close though to needing 91, IMO, for full potential but again it all depends on the program parameters and allowances (ECU).
I was happy when I first got the truck that it only needed 87 and non-synthetic fluids (except the diff, iirc on the M226), then I read about the older ones recommending 91 but being able to run 87, so thought maybe this 2011 was the same and 91 would allow full potential. Then I ran across that thread and also haven't noticed any difference since I've been running 91, so think I'll go back, as 20 cents a gallon adds up if it does nothing.
Of course, once you change the tune with say a programmer, then 91 would be beneficial for sure and more worth it. One thing that pisses me off is this ethanol BS!!!
Oh yeah, keep in mind after the engine ages with mileage and esp. low speed, stop and go, etc., it will build up carbon in the combustion chamber (no matter what type of gas you use, it's the nature of it) and then it could require you to run higher octane to prevent detonation, since it can get hot enough and stay hot enough to fire it prematurely before the spark...Add to this that crappy ethanol that can gum up the fuel system and we're constantly running against the gauntlet trying to keep these things running like new.
