pic: Two-speed low profile gearbox + PTO

[Aaron Ng]

As for the encoder, I do believe I need to add a gear and a shaft for that. At that point, wound't it be easier to mount encoders to the wheel shafts?

Perhaps you can add small gear on the output shaft. sdp-si has a ton of gears for your choosing. Broach a small one and you can fit it on the output hex shaft where you have a spacer. you might have to make a small plate to mount the encoder, else you cant mount the gearbox directly to the 2x1 (which is what i am assuming you are doing)

Or gear it directly to the CIM gear. Looking at your CAD, there seems to be space right next to the CIM. It would solve the problem of needing a dedicated plate just to mount a encoder, but it would make life difficult for your programmers.