For the 2017 to 2019 Rebel 300/500, you need to raise the tank to get to the ECU connection for the tach install. The 2020-up years is easier – they connect to the green wire on a ignition coil. They take a different Daytona tacho kit and harness.
For the 2020-up models, the harness directly plugs into the ignition coil (green wire).
2020 to 2022:
tach #17284 2020-2022 Rebel 300 – https://japan.webike.net/products/24200776.html
Daytona Velona 17283 (also 17284 $129.32) for “2020 – 2021” (actually 2020-2022) Rebel 300 https://japan.webike.net/products/24200777.html … note: I am 99% sure 17283 also fits 2020-2022 500 – Pedro Fernández Neri put one on his Rebel 500, a Japanese reviewer put one on his 500, plus I have seen other references that it fits the 500 (see Amazon Japan ad below).
2017 to 2019:
article on 17252: https://www.hondarebel3forum.com/threads/daytona-velona-60-tachometer-kit-installation-instructions-with-pics.14151/
https://www.jpcycles.com/2017-honda-rebel-300-cmx300/tachometers
Dayton Velona 92569 for 2017-2019 Rebel 300 https://japan.webike.net/products/24200775.html
Daytona Velona 17252 for 2017- 2019 Rebel 250/500 (same as Rebel 300/500 in US) …. (conflicting info) I saw another 17252 marked 2017-19 Rebel 250/300
I ordered this tachometer today for my 2017 Rebel 300:
DAYTONA VELONA Tacho Meter Kit Part Number 92569 – for 2017 to 2019 Rebel 300 … $105.71 USD plus Fedex $21.84 … arrives in 3 days
Below is Daytona Velona tach manual.
2017 Rebel 300 speedometer connector has 16 pins. Female connector has 5 pins missing. …. 2015-2017(?) CBR300 speedometer has 20 pins.
For my Rebel 300, I bought this one, but it didn’t work. It is $22 USD – much cheaper than a Daytona tach. It is for cars so I would need to seal it for wet weather. https://www.ebay.com/itm/164022590621
Generally speaking, a $20 motorcycle tachometer will not work on a modern bike that has an ECU, and is made for an older bike that has a CDI. As an experiment, try adding a resistor between the CPU and the tach – per Daytona, the wire from the ECU to the tach should show a resistance of 1MΩ.
Rebel500 