I currently have a 7 speed Shimano RD (rear derailleur) that is working perfectly with the stock 7 speed freewheel which is 14-28. Shimano has a MegaRange 7 speed freewheel that is 14-34 but the problem (for me) is I don't really like the huge jump from 34 to 24 teeth on it. I want another gear (cog) "sandwiched" in between those. I haven't found that in a 7 speed freewheel but some 8 speed freewheels have it (34-28-24...). So my question is, if I upgrade my RD such that it can handle the 34 tooth largest cog, can I reuse my 7 speed indexed twist shifter and chain and just not use the smallest cog? I don't really need the 13 tooth cuz since I am rarely in the top gear and the 2nd smallest cog is 15 which is good enough. My dropouts on my new MTB are already factory set at 135mm so there should be ample room for an 8 speed freewheel.
So my question is does this have a good chance of working or might there be problems? What pitfalls do I need to look out for? I made a spreadsheet of the gear ratios and they look very good. There is no large gap in the lowest 2 gear ratios and the middle gears that I mostly use are also very nicely spaced (between 11 and 14% gap).
From the combination of widening the cog ratios (from 14-28 to 13-34) and widening the front chainrings (from 24,34,42 to 20,34,42), my potential spread of gears will go from 3.5 to about 5.5 which is about 57% wider range.
From my riding experience so far, this will be a nice upgrade if it works.
Also, just recently I rode a friends bike that already has the MegaRange freewheel on it and it shifted fine. However, when going from the 24 to 34 cog it felt like it dropped 2 gears based on ratio. That is, I would prefer another cog in between the 34 and 24 such as 28 or 29. Upshifting from 34 to 24 feels like going from 1st to 3rd gear with a missing 2nd gear.