Blow the fucking dust off your purse strings and upgrade to the 405.
It has a Chafing Indicator.
I think the 405 just adds a color monitor, and maybe street maps. The real gusto is in the 605 or 705. Those have a feature that if you are riding with someone that also has one, you can see where they are (where they are on the course) on your unit. So neioghbor guy can tell where I crashed and went over the edge. Mainly so the family knows where to put the cross. Actually probably no cross, they'll just bring the dog out to piss on that spot.