wheel rotates, but robot moves slowly

I was trying to send cmd_vel command to a diff-drive robot in gazebo as in http://www.generationrobots.com/blog/en/2015/02/robotic-simulation-scenarios-with-gazebo-and-ros/ After some work, I was able to make two wheels rotate according to cmd_vel message. However, the robot moves extremely slowly. Is there any parameter I should tune to make it move normally? Thanks a lot