My LBS says most people don't measure their shoulders correctly. He says the proper way to measure shoulders is hands in a fist and hold arms out parallel to the shoulders. I was determined i needed 46's by measuring my shoulders bone to bone. I went in to buy a pair and walked out with 42's.

74 cm mountain bike
42 cm road bike
40 cm road racing bike
38 cm track bike

I am going through this right now and am experimenting with narrower bars after riding 44cm for several seasons. Most you want to know about the subject is contained in this excellent thread:
https://weightweenies.starbike.com/f...29754&start=75

GCN recently interviewed pro riders and at least among the world's best, narrow seems to be preferred 'independent' of rider size. Stands to reason a narrower bar is more aerodynamic and many find that a narrower bar is more comfortable by virtue of scapula position...how the skeleton supports the body. It is largely a myth that males vary greatly on ball joint to ball shoulder joint spacing differences...most difference is musculature as the ww thread divulges.

GCN vid on what pro's ride:

I think you are taking poetic license. Most people don't have a clue what they want or why and defer to an arbitrary shoulder measuring convention that has little proven veracity.

I was going to go back to a 42, also to avoid changing the second bike, but now I wonder if I should go straight to a 40.

Partly depends on your priority of speed...if you ride very fast and a priority. It may or may not be more comfortable. To me, trial and error albeit can be somewhat spendy, is always the best teacher. I believe most can acclimate to one size handlebar difference. The ww thread is an interesting read because many even bigger riders are going to 36 and 38cm wide bars. Some pros ride this narrow across the hoods as well. I am about to try 40cm C to C in hooks and ~38cm in hoods...and been many years before I have run this narrow but believe I will be pretty happy with them.

I honestly place little legitimacy in alignment of shoulder joints with bar spacing...just like mountain bikes offer very wide bars for leverage.

I solo ride 90% of the time which basically means the only racing I do is against is myself. I'm usually trying to better a PR or two on every ride, so speed does come into play. I spend a lot of time in the drops and the last bar I tried was an aero carbon bar with unwrapped flats. I think going with a 40 would benefit me in some ways. However I also climb a lot and spend an equal amount of time with my hands on those flats. Since that duty usually falls on one bike, I think I can play with the faster bike and go with the 40 bars. I'm starting to accumulate bars though, we need a handlebar trade thread like the saddle swap thread!

Because of the climbing you do, bar width 'may' be a bit more of tradeoff. Probably only way to determine is to try. I believe the ww thread and the video interview of pro riders debunks some of the misconception about handlebar width.
Good luck in your search.

Agree. The 40 cm bars that I'm riding are a little less comfortable than wider, but worth it for the more aero position.

The argument about breathing being constricted by narrow bars is answered by time trials where folks are able to go all out with their elbows practically touching.

I haven't watched the video yet but the WW thread has been a good read. I'm probably going to buy another cheap bar in 40 to test it out. Thanks for the help!

My pleasure. Since this thread re-emerged, I thought I would share a bit since I am re-evaluating bar size as well.