Gear ratios and Skid Patches November 12 2014
If your considering changing your gear ratio this gear_ratio_skid.xls chart will make the choice easier.
Picking the right ring size for street riding involves a lot of calculations. In general I find that I can choose between three sizes of rear cog without changing chain length. Based on a gear ratio and a quick rear cog change per environment I try to limit my gears to suit that chain length.
I created this excel spreadsheet a few years back and still find it handy with the math today: gear_ratio_skid.xls.
In particular tyre size makes quite a bit of difference to the gear ratio so I have several tabs down below in the spreadsheet to choose a page per tyre size.