To read the Speed of your car you need to enter this command on OBD-II dongle devices:
Then the module will respond something like that:
7E8 03 41 0D 4A 4A
The first value (7E8) is the ECU address in the CAN Bus (11 bits at 500k bauds).
The second value (03) is the amount of data bytes returned.
The third value (41) is the acknowledge of command received (01 + 40), this 40 is a constant added to all commands.
The forth value (0D) is just a copy of OBD PID that we are interested.
The fifth value (4A) is the data of speed that we want to know. This value is in hexadecimal, like all OBD-II data. In our case 0x4A = 74, it means 74 Km/h.
The sixth is just a copy of fifth, you can modify the configuration to avoid this copy if you want.