The change of tire will change the accuracy of the speedo because of the tire size. A larger tire will make the speedo read slower and a smaller tire will make it read higher.
Here's a site that lets you calculate the tire size and speedo diff
Going to a 255/75-16 from a 245/75-16 will make your speedo read 1.9% slower than it does today at the same speed.
2010 B4000 4x4 SE+
mods:Tinted windows,Line-X,BAK Rail Caps,rust proofing, stebel natilus air horn,ScanGauge II,hitch reciever cover, Belkin Iphone mount charger, rubbermaid floor mats, hose clamp tailgate, mud flap delete, Cargo light 3rd brake light, explorer lighted visors, mustang dome light, fog light mod, Mountaineer ohc, bosch evolution wipers, GenK51a mirror,Kenwood p680c speakers, Underhood LED's, Ranger winter rims 245/75-16 Blizzak DMV-1's, 180lbs winter bed weight