What is the difference besides the 2020 being a slightly weaker spine?
  Does anybody know the gpi of the 2020 Legacy shafts?

The 2020 is 13.5 gpi, and the 2117 is 12.0.  The 2020 has a deflection of .426 and a spine rating of 77#.  The 2117 has a deflection of .400 and a spine rating of 81#.  That's Easton's data and the spine weights are measured on 26" centers.

The first two numbers in an Easton arrow size denote outside diameter of the shaft.  20xx means it's a 20/64" (5/16") shaft.  21xx equals 21/64" diameter.

The second two numbers denote wall thickness.  The 2020 has a thickness of 20/1000" and the 2117 is 17/1000" thick.

The 2020 will make you a little heavier shaft, but the 2117 is a tad stiffer, owing to the larger outside diameter.

That's great, exactly what I wanted to know. Thank you very much.
  Sounds like the 2020 is what I'm looking for then, smaller diameter and heavier! perfect.

The 2020 shaft is my favorite alum. shaft. It flys well out of my 64lb. Pete George longbow as well as my 55lb. Bear TD. Years ago when we did not have many choices for shaft size 2020 was one of the most popular for hunting.

I have a few dozen 2020's,great heavy shaft.One of my favorite aluminum shafts.

Excellent response Curtis...completely correct!

I use both shafts, but am more attracted to the 2020 for the same reasons Dave and Thomas are.  The 2020's fly well through a range of bows and bow weights (for me anyway) and shoot particularly well through my 57#@28" Silvertip.  I like any shaft that is a smaller in dia. than the ferrule of the broadhead, and the 2020 fits that bill.  The 2117 is great shaft as well, is the 'old standby' and has been used for taking game for thousands of years      

Regards - Kris

Ok, so I am pulling 28" on my 55# Howatt Mamba and I get good flight from 2117s but they may be a tad stiff and I'm looking for a little more weight so the 2020s sound good. Now what about my 45# Howatt Hi-Speed, I'm thinking the 2016? Sounds like it would be a good younger brother to the 2020s. Same outside diamter but a little thinner so weaker spine? I wonder what the gpi is on the 2016.

The 2016s are 10.6 GPI and fly very nicely out of my 45# Martin Hunter

You might get away with 2016's on a 45@28 bow, but 1916 would probably be easier to tune.  2016's are 10.6 gpi, and 1916's are 10.0 gpi.

2020's are probably still a little stiff for your 55@28 bow.  You could probably get 2018's to tune good if you leave them a little long and shoot at least 150 up front.  2018's are 12.3 gpi and spine 67# with a .464 deflection.

My experience with 2018's and 2016's in the weights you're shooting has been good.  I've learned the hard and expensive way about shooting arrows that are too stiff...