Apple Watch: Workout app not using speed sensor

Hello KillmyFear, welcome to the Apple Support Community!

To use a third-party speed sensor (Bluetooth) with the Workout app on your Apple Watch Ultra 3, you must first pair it directly in the watch settings. On your Apple Watch, open the Settings app, scroll down, and tap Bluetooth. Wake up your speed sensor (by turning the bike wheel) so that it appears in the list of available devices under "Health devices", then select it to connect it. Once paired, the Exercise app will automatically use the sensor data for your outdoor and indoor cycling sessions. Have a nice day :)

This is because outdoors, your watch always gives absolute priority to its GPS to calculate the speed, which blocks the data from your sensor. Indoors, the GPS cuts off and gives way to the sensor. To fix this, open the Settings app on your Apple Watch, go to Exercise and activate the Energy Saving Mode before you go out, this will calm the GPS and force the watch to listen to the wheel of your bike. Tell us again!