Bill Gates told me a way to self test your max HR.
To quote him
Have a few days of rest before trying this, then give it a go. Secondly get a resting heart rate measurement by wearing the garmin to bed then as soon as you wake up, turn on the garmin and take your reading, getting up and moving about will raise your HR, best to take it as soon as you wake (or you could do it manually by counting your pulse). Adjust your zones accordingly. Roughly, (MHR-RHR)/6 gives you the boundaries. The garmin should allow you to modify your MHR and/or the zone boundaries, at least I'd hope it does since its an expensive bit of kit.
Then, start to read the time in zone statistics rather than average HR over your commute. You should most likely should stay within zone 3-4, will burn fat, give you an aerobic workout without making you ache too much. Wont hurt to throw in the occasional interval at a higher % though. But if you do, go easy the day after.
I dont know if you already mentioned which garmin you are using, if not could you share which model you are using and your opinion on it. Im in the market for buying one! Looking at the Edge 500. I was going to buy just a HRM but to get a decent one with programmable zones and workouts (intervals etc), its looking like its going to be worth a punt just jumping up to an all in one system.