I had the same question now that I'm getting ready to replace my stock BFG Rugged Trails (265/70-16) with Michelin LTX M/S (LT265/75-16). While I didn't really like the BFG's (too slippery/squeally), I have about 38,500 miles on them so I can't complain about durability.
I think the real reason Toyota (or any other mfg for that matter) doesn't want people to modify the speedometer reading for different size tires is that some people may change it just to lower the mileage being counted on the odometer, regardless of what tire size they have.
Like others have said - when you get larger tires, you are in fact reducing the mileage that your odometer/speedometer registers. According to the specs for the above tires, the revolutions per mile goes down from 680 to 652. Technically this could raise the vehicle re-sale value a bit since the mileage shown on the odometer would be less than the true mileage the truck travelled.
It may only be 3.24%, but that's an extra 3,240 miles your truck really travelled for every 100,000 miles shown on the odometer. Another way of looking at it is when your speedometer says your travelling at 65 mph, you're really going 67.2 mph.
I went through this before when I had two different sized sets of tires for my 4Runner. I made a little sticky note with two columns: "Speedometer Reading" and "True Speed" and filled in half-a-dozen values to remind me I'm going faster than what the speedo says when I had the larger tires on the truck.
People should also factor this in when planning routine vehicle maintenance which is usually scheduled by mileage.
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