Is an optical encoder wheel feasible to attach to the 3pi? It would measure average speed and distance traveled. A log of the info would be presented on the LCD after the run. I don’t even know if such a wheel/encoder exists for the 3pi size. Just thinking 
donde
There’s not a Encoder module yet for the 3pi so you’ll need to do some building. The best way to put encoders in your project depends on what techniques you are most comfortable with.
An aside here, the voltage to the motors of the 3pi is a constant 9.25 Volts because of the switch-up regulator on board. Are you not able to achieve a decent level of dead-reckoning accuracy without the encoders?