The Speedo DRD and the Speedo Healer both do the same thing -- they simply intercept the signals coming from the speedo sensor and send out signals that are whatever percentage you set faster or slower.
It's a simple percentage alteration -- speedos are normally off by a certain percentage.
The correction widgets come with instructions for calculating the correction needed, and there's even a calculator on the maker's web sites. The easiest way by far is to go for a ride with a GPS installed and note the differences in the readings.
2002 Suzuki DL1000 V-Strom, Dark Metallic Space Blue
1983 Suzuki GS850G, Cosmic Blue
2005 KLR685, Aztec Red - Turd II.2, the ReReTurdening