Check out SUPERLIFT! They offer the "Superlift TruSpeed Speed Signal Calibrator". It's the only speed calibrator out there for the Dodge Rams:
http://www.superlift.com/
It's a 4-wire hook up. POWER, GROUND, SIGNAL IN, and SIGNAL OUT. You just solder the unit inbetween the speedometer sensor wire. The only difficult part would be to locate the correct wire!

:
The best feature is that it is fully adjustable, so if you ever go to different size tires, or lower R&P gears, this unit is fully compatible with any tire and gear combination!!! :bigthumb:
_________________
1998 Dodge Ram 1500 Quad-Cab SLT 4X4 SB, 5.2L V8, 5" SkyJacker Stage II Kit, 35/12.50/15 Good~Year MT/R's on 15X8 Weld Racing Sidewinder Wheels, Flowmaster Cat-Back Exhaust System, Custom Audio System.