Garmins and Hill Grades

I was on vacation for the past two weeks (republic of panama)...i did a bit or running around the place I was staying and found a route in which the hills were sooo steep, I had to walk up them. The town I ran to was up in the mountains.

Some history on me: I'm a fairly decent runner and normally won't walk up hills on a run. Even if I have to jog really, really slow, I don't walk up hills.

Curiosity got the better and when uploading the data to my home computer, I looked at the hill grades. there were 3 major changes in elevation and the biggest one came in at a 51% grade! The other two were 47% and 39%. Based simply off eyeballing the elevation, at the time i thought the grade to the tallest hill had to be around 45%

So, my question here is, how reliable are Garmins (i have a 305) in determining the steepness of a route?

and how the he|| does a truck or car go up/down a 51% grade hill without making you wonder how well the brakes work?

and lastly, what are some steep routes you've run?

when i go back to panama next year, i plan on running that route and not walking it!