Tire and Wheel size upgrade?

[Big Red 350]

It's time for new skins (265/70-17's) on the 350. My local Ford dealer has some brand new 275/70-18"s on 18" 2007 spoke alloys that came off of one of their new F250 conversion trucks for a little over what I'd have to pay for just a set of replacement tires. My question is what does going up a tire and wheel size do to the truck's brain and speedo? I have a programmable tuner. I figure this is probably a no-brainer but I wanted to be sure from "truck authority" forum before dropping $$$. Thanks

[redstroker_2002]

u might have to re cal it. ur superchip should be able to do that for ya