This Android and Wear OS speedometer app has a free demo version: https://play.google.com/store/apps/details?id=com.alwaysonapps.alwaysonspeeddemo

The application shows the following metrics while keeping the device awake:
• Speed in km/h, miles/h, m/s, or knots.
• Average and maximum speed for the last 60 seconds.
• Distance traveled in kilometers.
• Duration of travel in minutes and seconds. Measured only when speed > 2.5 km/h.
• Current time in hours and minutes.
• Height above sea level (altitude) in meters or feet. If your smartphone does not have dual-frequency GPS, use the barometer-based altimeter option.
• Gradient in % (elevation gain divided by distance gain), a measure of inclination/slope. Averaged over 30 meters and 10 seconds.
• Only on smartwatches: Heart rate in beats per minute. Accuracy: normally very good; restart the sensor if way off.
• Battery percentage.
• Direction of movement (heading) in cardinal points and degrees. Shown only when the speed exceeds 4 km/h.
• The History tab shows the max and average speed and heart rate values for every minute of the current activity.

Gesture control:
• Swipe left to open Settings. Swipe left again to open the History tab showing the log of current activity.
• Swipe right to set the speed limit**
• Swipe up to change the font color to white and increase the font thickness for maximum readability under direct sunlight. Expect a higher power consumption.
• Swipe down from the middle of the screen to check the power consumption rate.

Text-to-speech:
• The app can read the speed aloud when it exceeds the speed limit.
• Touch the screen with your nose or chin to hear the current time without taking your eyes off the road and/or both your hands off the handlebar.
• Most languages are supported.

Important information:
• On Wear OS: Disable "Auto detect workouts" in Samsung Health and "Mirroring" in Google Maps to enjoy uninterrupted experience.
• Re-calibrate the barometer when the weather changes (sunny → cloudy, temperature change, etc.) for accurate altitude data.
• Please make sure GPS is enabled in the settings of your smartwatch before launching the app.
• The app automatically uses location data from the phone (instead of the watch) when it's available for maximum power efficiency and accuracy. Do NOT disable GPS on the watch in either case.
• The accuracy depends on cloud cover and your speed. The accuracy is very high when the sky is clear and your speed exceeds 20 km/h or 12 mph.
• The app does not store your activity data and does not connect to the Internet.

Settings:
• 17 different settings to customize your experience.
• Choose the metrics you want to see.
• Speed and HR max limits: get alerts when your speed or HR is too high; adjust the sound volume, vibration, and text-to-speech settings for alerts.
• Unit of speed.
• Screen rotation: rotate the text on the main screen until it aligns with your eyes. For example, if you wear your watch on your left wrist, set the Screen Rotate setting to 45° when riding a bicycle (either flat-bar or drop-bar).
• Inactivity alert.
• Screen brightness (on devices supporting rotary input)

Additional information:
• The display stays on until you quit the app.
• Numbers move up and down the screen at the start of every minute (e.g., 11:00:00, then 11:01:00, etc.) to prevent screen burn-in and give you a better sense of time.
• Green font has lower power consumption.
• Palm gesture rejection prevents you from accidentally turning off the screen.
• Power consumption: ~10%/h as measured on a 44mm Galaxy Watch 6.

Footnotes:
* This app uses GPS data instead of a magnetometer (compass) to accurately determine the direction of movement.
** Only on devices that support rotary input via a rotating side button or bezel. Alternatively, speed limit and sound volume can be adjusted using arrow buttons in Settings.