On Tue, 6 Jan 2009 04:47:13 -0800 (PST), Mikepier
wrote:

The previous owner of my house had put in a 3" dryer outdoor vent. Why
I don't know, perhaps this was the size years ago, but in any event I
thought I would compensate for this by running all rigid duct from my
dryer to the vent. I ran about 8 feet of 4" duct with 2 elbows: one at
the dryer outlet in the back, and one at a 45 where the duct meets
the outdoor vent. Then there is a 4"-3" adapter at the outdoor vent.
Being the fact that I ran all smooth duct, is this enough to
compensate for the 3" opening to the outside, or do I still need to
get a 4" vent?

You'll get better air movement with a 4" vent. I was not aware there
are 3" dryer vents, but I've seen plenty of 6" vents.