Tire Size Change Question

[stryker6040]

2008 Screw 5.4 went from tire size 275/65R18 to LT315/70R17. I first used the formula in the owners manual to calculate the correct number and by doing this I got 2741 so i programed this in took a 25 mile drive to the parents and used two higher end gps to verify speed and basically i had to show 75 with the speedo needle to go 70 according to both gps. at my parents i did another calculation this time taking the overall diameter of 34.7 and multipling it by 25.4 to get 881x3.1415 and got 2769 and then programed that new number and the speedo was still off by 5 according to both gps

Am i not doing something right or is the programmer not making the correct modifications to the cpu. any help would be great