It depends on the make and model of your dashcam. Some record speed while others do not. The best thing to look out for are cams with GPS - this is usually the basis for recording speed and timestamps based on location.
For example, the AKASO V1 with WiFi can record speed thanks to its GPS. The data is easily accessed via your Wi-Fi-connected smartphone.
You can, however, usually turn speed monitoring off if you do not want any data associated with how fast you are traveling. Check the dashboard camera reviews to find more details about speed recording.