Trying to remedy a rube goldberg duct work system that results in, ...,
less than optimal air flow to the far end of a run. What I can determine
by eye, tape, and feeling around up inside the duct looks like this:

* GMPN080-4 furnace/AC, plenum & duct work originally serving 1288 sq ft
* All original branches used top take offs & 6" flex pipe - all work fine
* Two long runs into garage/workshop area were added later - 576 sq ft
* Each run to 3 4x12 registers spaced above windows & along outside walls

The concerns I have with one particular run are all the twists and turns and
I know thats killing the air flow. I make it about 65 feet but I know each
change in direction makes it seem like more. How much more I don't know...

* Top take off to 18 ft of 7" flex pipe between floor joists
* Then to a metal 90 and 5 ft of 7" flex pipe across the bottom of the joists
* Then to a metal 7" round to 7" oval straight boot
* Then to 2 vertical 45s 7" ovals to get back up inside the joists
* Then to a 90 and 10 feet up inside the wall cavity with 7" oval
* Then to another 90 to get horizontal again
* Then to a 7" oval to 7" straight boot
* Then 24 ft (or so) straight 7" metal pipe with 2 of the registers
* Then a 90 to 8 more feet of straight pipe with 1 of the registers

I can't see any other way to have done the ducts given the constraints of
joists / walls / register locations. I can see 3 different things to try:

1) get rid of the flex duct and go all metal (reduce resistance ?)
2) install an in duct aux fan in basement and/or out in work shop
3) boost the main fan speed

#1 appears easiest/cheapest thing to try but will it help ?

#2 might be exactly what is needed but I really have no idea.

#3 is beyond me. I have no manual & no idea how to check it but I've read
that this unit has 3 selectable speeds. I'd really hate to pay a pro $100
or whatever to come out and flip a dip switch. If anybody has the book on
this unit...

Any advice from a pro/handyman/DIYer welcome.
--
Scotch.