Don't overlook the friction from the der pulleys or the chain. Freewheel friction is easy to discern, remove the chain and spin the wheel then touch the cog and feel the freedom (or not) of the freewheel's spin. With the chain installed spin the cranks backwards and note the time/revs it takes for the cranks to stop. Then try it again with a little pressure on the rear der cage (meaning the chain has a LITTLE bit of droop). Many times I'll see a major difference of back spin between these two chain tensions. Check your pulleys but don't be surprised if the chain itself is the main cause of friction. Andy.