Edited to say that I did notice a difference in switching from a "38mm" tire to a 40mm tire. I didn't think handling/traction/comfort wise it would be that big of a difference but it is. If you go to a 42 or 44 you will notice an improvement if you want that wide tire ride.
I've thought about this as well, I just got a second 700c wheelset for cheap but maybe I should have gone for 650b. However my bike has a 61mm BB drop and is built around the ability to use both wheel sizes. So as others point out maybe your bike isn't a great candidate for it. Anyway here's my thoughts about 650b...
Going from a tire marked as 38mm(but 40mm measured) to a 40mm marked tire the width was the same but the diameter seemed to increase. It's made my over all gearing a touch higher then I need for gravel; and the effort needed to accelerate the tire vs a 32mm seems higher.
So maybe I should've gone with 650b? Lighter, stronger tire then 700c; or I could've gone with a 24h spoke count and had comparable strength as my 28h 700c but even lighter weight. Lower gearing. Faster acceleration?
I've heard that 650b is "slower" or feels "slower" but at these sizes a 40mm 700c takes a bit of effort to stay rolling as well.
Last edited by GrainBrain; 03-04-21 at 06:22 AM.