pengyou

I have seen some motors for sale that have gears built into them to reduce the speed to about 300 rpms. I have also seen people use a common motor - speed typically 2,500 rpms, give or take and use a pair of pulleys or gears externally to lower the speed to something more manageable.

What are the advantages/disadvantages of each?

cerewa

Since no expert has posted, I'll try to answer the question. All types of gearing involve an efficiency loss; usually something like 99% or 97% of the power you had before gear-reduction is available after gear-reduction for each level of gearing used. It partly depends on how clean it is. There isn't a huge difference between the efficiency of gear-on-gear vs gear-chain-gear transmission, but gear-chain-gear is obviously what you want for moving power from the middle of the bike to the rear wheel. gear-on-gear setups can fit into smaller spaces-- that makes them practical for being completely covered up where they will stay clean, which helps for efficiency and reliability. If you were going to use a chain so that your motor can be somewhere besides inside the wheel and you need gear reduction, you may as well use different-size cogs to get gear reduction out of your chain drive.

It's mostly just a matter of what is most practical/convenient.