I went through this search a couple years ago. I found that Harley Davidson used to make a speedo/tach combo that had the speedo on the top half and the tach on the bottom half. I wanted one of these because the tach was big enough to be easy to read, but they didn't make them anymore.
I ordered one of the Medalion brand speedo/tach gauges from a catalog. When I got it I realized it wasn't going to sit on the tank exactly like the original, and it was not exactly plug and play. I sent it back to them.
I ended up getting the newer version of the Harley Davidson speedo/tach gauges. It's a little hard to read the tach at first because it is so small.
But, I didn't get it to tell me when to shift, I got it to get an idea of what rpms I was at on the highway at different speeds.