Originally Posted by FMKeith
My truck is at Ford right this second getting updated. I will let you guys know how it goes.
Tire Size is the issue
The Navigation system uses a combination of inputs to accurately locate the vehicle in all conditions. The input from the GPS satellites is the primary input; however the system also looks at the wheel pulse from the ABS module to calculate distance along with the gyro information from the RCM to calculate direction. This allows the system to continue to function in conditions where there may be a loss in signal (such as in a tunnel).
Altering tire size affects the count of the wheel pulse because the system will calculate distance inaccurately - for example the system expects 100 yards based on the count, but the vehicle actually traveled 110 yards because of the larger tires. When viewed on a map, this will place the vehicle off the road or contribute to a mis-timing of corners.