This module is necessary to correct the OEM speedometer readout when the final drive ratio or rear tire diameter has been altered. Can also be used to correct non-ratio modified models with the OEM electronic speedometer that read wrong straight from the factory. Push button up/down adjustments make it easy to set. Waterproof module measures 2 inches long x 1 1/4 inches wide x 5/8 inches tall and has 12 inch leads. Plugs directly into OEM wiring harness. Includes calibration instructions.