Mikey-
Not sure i understand my question also

But what I think I recently learned is that different size wheels require different sensors to relay info to their respective speedometers. Apparently, speedos aren't the main concern, their wheel sensors are the concern. Different sensors will register different mph on the same speedo. For example, sensors for a 19" and a 21" wheel would show a different mph on the speedo. Another difference besides being cable driven or electronically driven, is whether it's configured to fit in a tank mounted dash or a holder in a handlebar mount. Also, wiring is a concern because some speedo models include wiring for self canceling signals, for one example.
The reason I initially asked the question is that I am attempting to relocate the gauges of my FXR3 to a handlebar mount, so i don't have to look down at the tank while riding, if i wanted to check them. I also want to preserve my FXR3 tins, so figured I could kill two birds with one stone and switch to an FXLR tank (smooth tank with gas cap on the right). However, I discovered that the stock FXR3 speedo because of it's configuration is too deep and will not fit into a holder of a handlebar mount. So, I started searching for a speedo which would fit. During my search, I discovered that speedos were described in ratios, which confused me. What I didn't realize until someone told me an hour ago were the reasons (above) why certain speedos will work properly and why some will not. I was looking at it from a different (wrong) perspective, which is the reason I asked an ambiguous (dumb) question.

I am almost there now. I know the characteristics/specs of the speedo I need, however, I still don't know the p/n for it.