On my 01 4X4 is came stock with 245/75's. I went up to a 256/75 and it changed my speedo to being 2.5-3 MPH's slower. So the speedo was saying I was going 70 and in reality I was going 73 or so. Check out this site to calibrate his tires. It might help, I hope! Here's the site...Tire Calculator
It doesn't help in how faster the truck is going with the bigger tires on but it does say how many revolutions per mile the tires spin. I had to use this conversion to update my computer when I put the 265's on. I have an X3 and you can change the RPM setting for the tires to correct your computer/speedo. However I am not acquainted with the superchip one and how it can correct tire sizes.
-2011 F-150 Lariat: 5.0 "Coyote" V8, 6-spd Auto (6R80E), 3.73 ELocker. White Platinum Metallic Tri-coat.
-1992 Ford Ranger: 4.0L OHV, 5spd (M5OD-R1), 3.27 Open.