You don't need the gps bim for the DD speedo to work. The kit comes with plug you use to replace the old speedo cable. You patch into your current VSS sensor and that signal is used to calibrate the speedo.I was thinking about the DD unit
with all the hills, trees, everything else, doubt it would be worthwhile for me. I can't even get a cellphone to work properly most of the time, even when using the phone gps
also have a garmin unit I use for hunting and hiking and ATV'ing. Rare to get a good reliable signal unless on a hill top.