4WD robot chassis

I'm looking at a Hitari Tumbler radio-controlled car chassis.  It has four wheel drive, with front and rear wheels on each side geared together, and to the motor.  There are, of course, two motors, for left and right side drives.  It's old, and missing the bodywork, battery and radio transmitter -- but we won't be needing any radio!

Next step is to get some kind of sensor on it for odometry (measuring how far it has travelled, by sensing wheel rotation).  But that's not going to be easy, if I want high resolution.  I need something like 200 sensor pulses per wheel revolution, so that I can get good control of the motor(s) at low speed.  So far, I have a really neat way to get about 20 pulses per wheel rev., whcih isn't really enough.