Will adding approximately one inch diameter to the tire size affect the speedometer (going from 265/65-17 to 265/70-17)? Or, is this even measured from the tires?

You will see a small differance.. I would get the speedo recalibrated just be safe.

I made the exact switch you are concidering. According to my GPS, the speed is now 63mph while indicating 60. My speedo used to match the GPS perfectly.