[Joel J]

figure out how many counts per rotation for the GTS..

Put the robot up on blocks..

Run the robot at max speed for 5 seconds..

record the final count of the GTS..

rough_speed = GTS / counts_per_rotation_of_robot_wheel * robot_wheel_diameter * PI / 12 / seconds_robot_drove_for

that a number in feet/sec

If you want a live speed, then you can take the current GTS count and subtract from it the GTS count at the last cycle, then apply to that number the following formula:

rough_speed = delta_GTS * robot_wheel_diameter * PI * 38.1 / counts_per_rotation_of_robot_wheel

that's a number in in/sec, for a bit better resolution.

BTW, everything other than GTS and delta_GTS can be calculated and combined before hand. That is, they are constant.