My Garmin strap has gotten loose as it got older and lost elasticity (bought it with my old 305, now used with 500).

The strap needs to be pretty tight to make good contact and get a good reading. I can tell when it is getting loose, I start getting erratic readings, even with a new battery.

I adjusted the strap as short as it would go. After a while, even that wasn't enough, so I shortened it more by sewing a tuck. I need to break down and buy a new strap.