I cracked my rear derailleur pulley on my commuter bike so I bought a new set. Who knew they came in 10 or 11 tooth sizes? Not me. And did I get lucky and get the right size? Did I notice only after installing them? And do they seem different than the Shimano Deore pulleys I'm replacing? Does my bike not shift as well as it did with the broken one? Did I save the broken one to put back? This should be so simple, but somehow I'm making it into rocket science. Don't mind me, I'm just busy spending money and making my bike worse.
UPDATE: New 11 tooth pulley arrived. Installing it seems to have cured all shifting ills. And I have spare 10-tooth pullys for future road bike replacement.