Unfortunately an accelerometer can only sense acceleration (the inertial force felt when the velocity changes). Moving at a constant speed gives a zero reading. The best way to measure speed is to fit a rotation sensor to your wheels. This may deliver an angle directly, or you may need a sensor counting pulses as the spokes of a wheel go by.
The change in angle will give you the distance travelled (multiply by the circumference), and dividing by the elapsed time will give you the average speed. Keep repeating your measurements for a continuous readout. You might like to look at the new “meters” extension to show your speed in cm/sec as a 2-digit readout.