I have a Maillard Helicomatic 6-speed freewheel (original) on my 1984 Trek 560. The cogs go from 13-24 (not sure what the inner 4 cogs are - haven't removed the freewheel yet and counted them), but I would like to 'tighten' this range to something like 12-19 as i ride most flat areas. Any ideas on a good cog range that would reduce the number of cogs I need to buy. There are several individual cogs on ebay. Anyone had any experience with this change? Thanks!