See posts 181 and 185 and I think you'll have SOME of your answers. I have the OEM size up front and 285/35s on the back.
Look over threads here in "Wheels, Brakes, Tires" and you will catch on. This, as John pointed out, has been discussed before.

From all indications, the speedo/odo is driven by the sensors in the FRONT wheels, if they stay OEM, fine.
I have 285/35s in back and have seen lower RPMs and tha'ts fine with me. (If I want to move out faster, I just shift a bit later than before.)

But it's way past my bedtime, I'm out!