Speedometer calibration is now an electronic function within the truck computer.
You can recalibrate for differing tire sizes using a programmer, such as a SuperChips or HyperTech unit. If you have a HEMI, I'd suggest a DiabloSport Predator.
One other option. Since Dodge now makes and sells trucks with 20" wheels and tires, they should have a flash for this in their StarScan tool. When I upgraded tires and wheels on a GM truck some years ago, the dealer was able to load a program that accounted for the larger size using their "ScanTool II" handheld programmer and code reader.
I'd check with the dealer service manager or service writer on the availability of a flash for your computer on this.