In both my 60cx and vistaCX there is a trip odometer and an odometer. I always reset everything from the trip computer page, for there all you do is hit menu, and select reset. you will then get check boxes for what data you would like to reset.
if you reset trip data it will clear the trip odometer, or if you reset the odometer it will clear the main milage ticker.
from this menu you can also clear waypoints, routes, saved tracks, tracklog, max speed, and elevation data if you have the barometric sensors.

The way these GPSr calculate the odometer data makes it read slightly higher than actual mileage. if you want a more accurate mileage reading use track log data.