I have stock 265/70R17 goodyear rts and I'm going to run nitto terra grapplers 285/70R17. How much power and torque will I lose from such a transition? Will it hurt my transmission when I pull a lot of weight?

 

I doubt you will notice a difference. They are only an inch taller.

 

What is the difference in circumferance going to the larger tire, percentage? You will be operating lower on the engine HP power curve. You will feel the power loss, expecially off the line and when towing a load.

You will also have reduced braking because the tires/rotors are turning fewer revolutions to stop the truck in the same distance .... the pads will be working harder.