First, I don't think the size is 265/75R17. I think it is P265/70R17. There are LT's in that "size".
Second, you don't need to inflate a Load Range E to 80 psi. In fact you can inflate to whatever pressure you want, just be aware that the load carrying capacity varies by inflation pressure, so choose the pressure appropriately. In your case, the obvious answer is 40 psi.
The max pressure listed on the sidewall has NOTHING to do with fuel economy. Rolling resistance is mostly controlled by the materials the tire is made out of - mostly the tread rubber compound and the amount of rubber in the tread.
Ergo, an All Terrain tire will have more RR than an All Season tire, all other things being equal (which most of the time those two types of tires also include a different tread compound, the AT having more cut resistance which will hurt RR.)
Also, a tire with a high treadwear rating will have worse RR than one with a lower rating, all other things being equal (and like the example above, those things usually aren't equal!)
Yes, it's complicated.
And just to add clarity, the pressure listed on the vehicle tire placard is applicable for ALL tires in that size, regardless of who makes the tire. You may find that the ride or the handling is different, but that is NOT because of the max pressure. It's due to the way the tire engineer designed the tire - and those things do not affect that load vs inflation pressure relationship.
In this case, I suspect that the reason a 35 psi rated tire is specified at 40 psi is for handling reasons. That increased pressure will obviously hurt the ride and is not needed for load carrying capacity.
I have seen that done before, and it always puzzles me why that happens - except for an 11th hour "fix" before the vehicle production starts.