So if I change tires from the stock 255/70 to 265/75 and change from 3:55 gears to 4:10s, about how much will this change my RPMs on the highway?

 

You are changing the tire size by 5%, but the gearing by 15%. So, your engine speed will be about 10% higher than stock So whatever speed you are turning 2000rpm now, you'll be at 2200 after the changes at the same speed.

Just to confuse you, though, the number will all line up (speed, rpm) until you change your computer to adjust for the new tires/gears.