2 guitars action measures same but feels very different
 

I have 2 new Yamaha red label guitars. One Fsx3 and fgx5. I felt like the action on the fs was noticeably higher feeling than the fg so I thought maybe needed a setup. I measured the nut and action they were both more or less the same. Relief is about the same as well. Was a bit high but I adjusted it on both.

How can the action feel so different when they measure very similar to the same? What am I missing?

Two reasons come to mind - they are enough off to make a difference. Secondly, do they have exactly the same strings? String tension/gauge can make a big difference in how a guitar feels.

They are both brand new so same strings and everything

a thought
 

OP needs to be SURE of the strings on both.

they're-both-new-so-they-gotta-have-the-same-strings is a weak assumption.

They both shipped new from the retailer so why would they be different? How would I check them?

A guitar can be purchased and returned with the buyer changing strings prior to return. The only way to assure that both sets are the same make, model and gauge is to change them both to the same strings. They are supposed to come from the factory with Elixir 80/20 Nanowebs, pick your gauge or use different strings of your choice.

I assume the frets are the same on each but if not .. I perceive higher frets as a higher action. Not saying there’s any sense to that but it happens, to me anyway.

Lots of variables…
 

The feel of a guitar can be affected by the many factors, not just the setup parameters, string type and gauge.

Two factors that appear to change the feel but are seldom figured into the equation are the responsiveness of the top and the height of the strings above the soundboard - not the same thing as the action. Instruments with a very lightly built, responsive top consistently feel easier to play in my experience. To a lesser extent I’ve also noticed that instruments with a high string height over the soundboard also feel stiffer to play. Typically this height is set to around 1/2” at the bridge, achieved through a combination of neck geometry, bridge and saddle height. In those stiffer guitars there has often been more string break angle over the saddle so this may be what is at play rather than simply the height above the soundboard.

From a physics point of view I’m unsure of exactly how and why these factors make a difference but empirically they do. Perhaps some of the more engineering-minded contributors to AGF may help to explain.

Did you ever notice that they have entirely different body shapes?

Sure you did, but maybe it didn't occur to you that they sit differently on your leg, under your arm, etc. THAT....changes the center line of how the neck meets your fretting hand, thus changing the amount of leverage required to fret a string.

The human body is so incredibly sensitive, but the logical mind often ignores this simple fact, hence it tries to measure what's happening instead of feeling what's happening.

Specs are often meaningless.

HE

With identical nuts, the string spacing may still be cut slightly different. Most players can feel a few thousandths difference.

Truss rod does not adjust action.

Action is adjusted with the nut and saddle.

Truss rod adjustment is in the mix, it's just not the main adjustment.

Necks the same shape and size? Also the body size of the two is pretty different and that affects what position you may be holding the guitar.
Frets the same height and width on the two guitars? And as already pointed out the strings may be different on the two guitars.