It doesn't hurt to have a backup. A wheel sensor type speedo will have a magnet attached somewhere ie brake rotor, and there will be a calculation needed for overall diameter or circumference of the tire to be programmed into the brain. When finished you'll want to compare it to a known working speedo. If you have a smartphone you can mount to the bars somewhere, download a Speedometer app and just use that.
Re the factory speedo, they are known to give trouble. Be certain your cable is working 100%. Not in a bind, plenty of lube, and not damaged -outer or inner. The connections at the speedo and at the angle drive.
does anyone have a quick link to the St. Vitus Dance on the forum?