Despite the engines being very similar, you can see there are some tell-tale signs that they are very different.
If you'll look here at the 2012 gx460 specs
http://pressroom.toyota.com/releases...+info.download
and here at the 2012 tundra specs
http://pressroom.toyota.com/releases...tures.download
you'll see that they do in fact both have the "same" 4.6 liters of engine. The differences start to pile up immediately though. For instance, in the tundra the bore and stoke is: 3.70 x 3.30 in. In the GX460, it is 3.70 in. x 3.27 in.
GX460 hp numbers: Horsepower (SAE) 301 hp @ 5,500 rpm
Torque 329 lb.-ft. @ 3,500 rpm
Tundra hp numbers: Horsepower 310 hp @ 5,600 rpm,
Torque 327 lb-ft @ 3,400 rpm
Also, we see that the tundra engine is Direct injection alone, wheras the GX460 version is MFI.
So despite both engine being of the same designation, the gx460 requires 91 unleaded because it was designed for it obviously. It's quite possible that the gx460 ecu is programmed more aggressively to take advantage of the higher octane and the tundra is not.
Take your pick of reasons that they differ, the point is if the manufacturer recommends premium it is because the engine was designed to and runs "better" on it than on regular, whereas that might be the exact opposite for the tundra.
I was in a similar situation before we purchased our GX470 coming from a 5.3l envoy. All in all we spend about 4-6 dollars more a tank and get the same or better mileage so I really wouldnt be that concerned about just buying the premium. Also, it probably wouldnt help to put regular in it anyway, because the ecu will probably detune the the engine accordingly to knock it may sense. You wouldnt know that its happened until after you got worse mileage....
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