Garmin 305 altitude accuracy? Like to hear experience.

The faster you move, the more acurate it seems to be with accumulated climb and descent.  I found it to be within a couple precent of what any kind of mapping program will give you for a bike ride.  When viewed, the profiles look similar to what I rode.

However, during a run, not so much.  I look back and see lots of peaks and valley, when it was a flat route, or a steady climb or descent.

It's not worthless, but it's not spot on.  For general ball park, it's great.  And I enjoy looking at the data and using it.

As far as changing routes I have had no luck with this or even planning a new route and uploading.  I have searched the web and it seems difficult.  You can redo the exact course but I am pretty sure you can't adjust the time.

Ditto this.  For biking the corrections are often modest.  For running, it often shows me climbing 2-4x what I actually did.

I'll disagree with this.  The garmin has both the gps and pressure sensor so, in theory, one method will correct the built in error of the other.

You mention two methods, gps and route tracker, one accurate , one inaccurate, what do you hold as the gold standard.  What do you hold as absolute so that you can call one accurate and not the other?

I haven't played with route tracker much, but IIm pretty sure the back-end is google maps and gis, which is the basis of other programs I have used extensively.  Take the route you create and output it to google earth and look at it in exagrated 3D and you'll see the route jumping valleys and cutting through the tops of hills.

If you use a gps on the same route multiple times and the ascent comes out close that would infer accuracy. (Now I'll have to go digging through my messy pile of datafiles and check this hypothosis)

I believe there's alot more variablity in the reported ascent do to the software you choose,  ie. load the same gps data file into four different programs and get four diffent, sometimes wildly different, answer.  This is from the the software using different algorithms to decide what is ascent and what isn't, ie how long/high must you climb before it's added to the ascent total.  Of course the same is true between various routing software.