Finally added the code for encoder counter using interrupt pins on Pins 18,19,20,21 on the Mega. But the encoder values are getting very skewed which makes these values unusable. So I have updated the code on Github with the encoder counting and storing in variable along with the inaccuracies. For my ROS robot, I will modify this robot with new motors with more accurate encoders, Also will need to fix any possible problems that might pop up but for now the RC Car seems to run. Waiting for the battery to come. Will wire it up and share a video of it finally. Since this is such a simple project, I won't be putting a full build guide, but will list out the exact list of components for anyone to experiment with.