Change in tire size will impact speedometer accuracy, larger tires will make actual speed higher than indicated.
You can change the drive gear to compensate. First, make sure the speedometer is correctly nulled, this is the needle at the zero mark with the truck stopped. The needle is only a friction fit on the shaft and it can be carefully adjusted with the dash removed. This will not change odometer calibration.
Once nulled, test over a measured mile at 30 MPH indicated, 2 minutes is the time. Adjust accordingly.