Gearing calculation - Gear Inches

Calculation of gear inches for drivetrains with derailleur and without derailler - eg. front shifting with Schlumpf Speed-Drive, High-Speed-Drive, and Mountain-Drive and rear shifting with Rohloff Speedhub, Shimano Nexus, and SRAM S7, and Sturmey Archer planetary geared rear hubs.

Your current bike
Wheel diameter Measure or look on side of tire	Select 20" 406 20" 451 26" MTB 700c Road	Wheel size:
Front chainring sizes Count the teeth	Small:	Large:
Rear cog sizes Count the teeth	Small: 8T 9T 10T 11T 12T 13T 14T 15T 16T 17T 18T 19T 20T -	Large: 21T 22T 23T 24T 25T 26T 27T 28T 29T 30T 31T 32T 33T 34T 35T 36T
SRAM internal gearing? (eg: 63-sp or 81-sp bike)
Gear range:	Low:	High:

PBW bike (default 20" 406mm wheels)
Wheel diameter	20" 406 20" 451 26" MTB 700c Road	Wheel size:
Front chainring
Schlumpf Drive	None:     Mountain-Drive:     Speed-Drive:     HighSpeed-Drive:
Rear cassette Cassette: 8T 9T 10T 11T 12T 13T 14T 15T 16T 17T 18T 19T 20T - 21T 22T 23T 24T 25T 26T 27T 28T 29T 30T 31T 32T 33T 34T 35T 36T Raw gear: - SRAM 3x7 /  Dualdrive:  - 14-sp Rohloff Speedhub 500/14 Cog: 13T 15T 16T 17T - 7-sp Internal Cog: 18T 19T 20T 21T 22T 23T 24T 25T 26T 27T SRAM S7: - Shimano: - Sturmey: -