When recalibrating the vehicle's speedometer, it helps to have a GPS available. You can pick up some pretty nice units that plug into the computer's USB port, however trying to find any decent software that either A) displays in MPH, or B) does not install a host of spyware... has been difficult. The one program I found that displayed what I needed had such a small window that there was no way I wanted to try reading it while driving down the highway.
If you could add simple GPS functionality to VCM_Scanner, and allow the info to be displayed on the graphical cluster, it would make comparisons easier (either by watching the cluster in real-time, or being able to compare logs later on). The two pieces of information that would be useful are actual speed (MPH/KPH), and distance driven (comparing odometer readings over long distances can allow us to dial in the speedometer exactly).
The GPS units I have worked with are all either serial connections, or have their own USB-to-serial converter built in. You just have to know which serial port and baud rate to use, and the NMEA data can be read directly.