I still own and operate a cc-6000 (bought it in '88). By pressing all three buttons simultaneously, the unit is cleared and mile/h is displayed. Press 'Start Stop' button to alternate between km/h and mile/h. Once you've made that selection, press 'mode'. Now you should see 216, the approximate circumference of your tire in cm. To determine a relatively accurate measurement of the circumference of your tire, measure the distance your tire travels with three tire rotations (more is better if you have a long enough tape measure). Divide this measurement by 3 and multiply by 2.54 if you measured in inches. This should give a number close to 216 as displayed. To change the number in the display, press 'Start Stop'. The number can only be increased, but by pressing and holding 'Start Stop' the number will increase rapidly and start over. Once you have the proper value, press 'Mode' and you're set to go.