I have the exact issue with this older post regard two motor with different. I found this older post
http://forum.arduino.cc/index.php?topic=100522.0
In the post, the word encoder keeps coming up. What is this encoder anyway? The motors I use are just the run of the mill DC motor, nothing fancy.
Google will be happy to explain motor shaft encoders to you.
Encoders measure position or angle, and often only record relative changes. They are
usually digital devices using 2-bit Gray code ("quadrature"), but there are many types
from a few dollars to enormously expensive ones used in top flight precision machinery.