getting speed for gearmotor

I need to get the speed for gearmotors from sparkfun. I am looking for speed in km/h or something similar. I am controlling the motors with PWM on the arduino. I am also using the 32x7mm wheels from sparkfun as well. Can someone help me out?

gearmotors:http://www.sparkfun.com/commerce/product_info.php?products_id=8911

wheels:http://www.sparkfun.com/commerce/product_info.php?products_id=8901

To get accurate wheel rotation speed you probably need rotation encoders on either the motor shaft or on the wheels.

With the information provided, you can only calculate the following:

PI * Diameter = circumference, which is also the distance traveled in 1 rotation. PI * 32 = 100.5 mm Circumference * RPM gets you distance travelled in 1 minute.

The motor lists 440 RPM @ 6V so: 440 * 100.5 = 44220 mm/minute with a steady 6v power source.

The problem is that I change the value of the voltage through PWM.

The problem is that I change the value of the voltage.

The multiplication would apply if it were 0 to 6v. Whenever I did my recent spirograph project I was taking the value of the maximum voltage and dividing it to read 255.

pwm 0 = motor 0 speed pwm 255 = motor speed full blast

With the right calculation you could get the speeds but getting those I couldnt help (been a while since math ;P)

thanks...