There's a lot of indispensable information on the late Sheldon Brown's website, but none is quite so technically ravishing as his gear inch/ratio calculator. We use it on a daily basis. Because of that, we have integrated it into the site here so it is always close at hand, but be sure to visit both the original gears page and Sheldon's site in general for a wealth of information.

Wheel Size	29 inch (nominal) 28 inch (nominal) 27 inch (nominal) 26 inch (nominal) 700 X 56 / 56-622 / 29 inch 700 X 50 / 50-622 / 29 inch 700 X 44 / 44-622 / 29 inch 700 X 38 / 38-622 700 X 35 / 35-622 700 X 32 / 32-622 700 X 28 / 28-622 700 X 25 / 25-622 700 X 23 / 23-622 700 X 20 / 20-622 Tubular / Wide Tubular / Narrow 28 X 1 1/2 / 40-635 27 X 1 3/8 / 35-630 27 X 1 1/4 / 32-630 27 X 1 1/8 / 28-630 27 X 1 / 25-630 26 X 2.35 / 60-559 / MTB 26 X 2.125 / 54-559 / MTB 26 X 1.9 / 47-559 / MTB 26 X 1.5 / 38-559 / MTB 26 X 1.25 / 32-559 / MTB 26 X 1.0 / 25-559 / MTB 650 x 38b / 38-584 650B 650 x 28C / 28-571 / 26" road/tri 650 x 25C / 25-571 / 26" road/tri 650 x 23C / 23-571 / 26" road/tri 26 X 1 3/8 / 35-590 24 inch (nominal) 24 x 1 / 25-520 20 inch (nominal) 20 X 1.75 / 44-406 / BMX 20 X 1 1/4 / 28-451 18 x 1.5 / 40-355 17 x 1 1/4 / 28-369 16 x 1 1/2 / 40-349 16 x 1 3/8 / 35-349 16 x 1.5 / 37-305
Crank Length	150 mm 152 mm 155 mm 158 mm 160 mm 162.5 mm 165 mm 167.5 mm 170 mm 172.5 mm 175 mm 177.5 mm 180 mm 185 mm 190 mm
Gear units	Gain Ratios Gear inches Meters Developement MPH @ 40 RPM MPH @ 60 RPM MPH @ 80 RPM MPH @ 90 RPM MPH @ 100 RPM MPH @ 120 RPM KPH @ 40 RPM KPH @ 60 RPM KPH @ 80 RPM KPH @ 90 RPM KPH @ 100 RPM KPH @ 120 RPM
Chainrings
Stock Cassette	Custom Sizes 9-speed "Century Special" 13-14-15-17-19-21-24-27-30 9-speed "Cyclotouriste 13" 13-15-17-19-21-24-27-30-34 9-speed "Cyclotouriste 14" 14-15-17-19-21-24-27-30-34 9-speed "High & Wide" 11-12-13-14-16-18-21-24-28 9-speed "Classic 9" 13-14-15-16-17-18-19-21-24 10-speed 13-14-15-16-17-19-21-23-26-29 10-speed 13-14-15-16-17-18-19-21-23-26 10-speed 12-13-14-15-16-17-19-21-24-27 10-speed 12-13-14-15-16-17-19-21-23-25 10-speed 12-13-14-15-16-17-18-19-21-23 10-speed 12-13-14-15-16-17-18-19-20-21 10-speed 11-13-15-17-19-21-23-25-28-32 10-speed 11-13-15-17-19-21-23-25-28-34 10-speed 11-12-13-14-15-16-17-19-21-23 10-speed 11-12-13-14-15-16-17-18-19-21 10-speed 15-16-17-18-19-20-21-22-23-25 10-speed 16-17-18-19-20-21-22-23-25-27 Capreo 9-speed 9-10-11-13-15-17-20-23-26 9-speed 11-12-13-14-15-16-17-19-21 9-speed 12-13-14-15-16-17-18-19-21 9-speed 12-13-14-15-16-17-19-21-23 9-speed 11-12-13-14-15-17-19-21-23 9-speed 12-13-14-15-17-19-21-23-25 9-speed SRAM 12-13-14-15-17-19-21-23-26 9-speed 12-13-14-15-17-19-21-24-27 9-speed 13-14-15-16-17-18-19-21-23 9-speed 13-14-15-16-17-19-21-23-25 9-speed 14-15-16-17-18-19-21-23-25 9-speed aq/ar 11-12-14-16-18-21-24-28-32 9-speed ap 12-14-16-18-20-23-26-30-34 9-speed as 11-13-15-17-20-23-26-30-34 8-speed "8K7-13-30" 13-15-17-19-21-23-26-30 8-speed "8K7-13-32" 13-15-17-19-21-23-26-32 8-speed "8K7-13-34" 13-15-17-19-21-23-26-34 8-speed "Megarange" 11-13-15-17-20-23-26-34 8-speed "ak/an" 11-13-15-17-20-23-26-30 8-speed "P" 12-14-16-18-21-24-28-32 8-speed "Q" 12-13-14-16-18-21-24-28 8-speed "R/ah" 11-12-14-16-18-21-24-28 8-speed "S" 12-13-14-15-16-17-19-21 8-speed "T" 13-14-15-16-17-19-21-23 8-speed "U" 12-13-14-15-17-19-21-23 8-speed "V" 13-14-15-17-19-21-23-26 8-speed "W" 12-13-15-17-19-21-23-25 7-speed Freewheel 14-16-18-20-22-24-28 7-speed "F" 14-16-18-21-24-28-32 7-speed "G" 13-15-17-20-23-26-30 7-speed "K" 13-15-17-20-24-29-34 7-speed "M" 13-15-17-19-21-24-28 7-speed "H" 13-15-17-19-21-23-26 7-speed Sram 12-14-16-18-21-26-32 7-speed "E" 12-14-16-18-21-24-28 7-speed "L" 12-13-14-15-17-19-21 7-speed "I" 13-14-15-17-19-21-23 7-speed "J" 13-14-15-16-17-19-21 7-speed "ab" 11-12-13-14-15-17-19 7-speed "ac" 11-13-15-18-21-24-28 7-speed "ai" 11-12-14-16-18-21-24 7-speed "am" 11-13-15-18-21-24-30 7-speed "MegaRange Cassette" 11-13-15-18-22-26-34 7-speed "MegaRange Freewheel" 11-13-15-18-21-24-34
Custom Cassette
Internal Hub	----- No planetary (internal) gears ----- SRAM/Sachs/Torpedo 3-speed, including 3 x (n), Dual Drive SRAM/Sachs/Torpedo 5-speed Shimano Nexus 4-speed Schlumpf Speed Drive bottom bracket Schlumpf Mountain Drive bottom bracket Shimano Nexus Inter-3 Sturmey-Archer AB wide-ratio 3-speed w/brake Sturmey-Archer AC close-ratio 3-speed Sturmey-Archer AF close-ratio 4-speed Sturmey-Archer AM medium-ratio 3-speed Sturmey-Archer AG wide-ratio 3-speed w/dyno Sturmey-Archer AR ultra-close-ratio 3-speed Sturmey-Archer ASC fixed close-ratio 3-speed Sturmey-Archer AT3 wide-ratio 3-speed Sturmey-Archer AW wide-ratio 3-speed Sturmey-Archer AWC wide-ratio 3-speed w/coaster brake Sturmey-Archer FB wide-ratio 4-speed w/brake Sturmey-Archer FC close-ratio 4-speed Sturmey-Archer FG wide-ratio 4-speed w/dyno Sturmey-Archer FM medium-ratio 4-speed Sturmey-Archer FM modified to 5-speed Sturmey-Archer FW wide-ratio 4-speed Sturmey-Archer K, KB, KBC, KT, KTC wide-ratio 3-speed Sturmey-Archer KS close-ratio 3-speed Sturmey-Archer KSW medium-ratio 3-speed Sturmey-Archer S3C wide-ratio 3-speed w/coaster Sturmey-Archer SBR wide-ratio 3-speed w/brake Sturmey-Archer S5 5-speed Sturmey-Archer Sprinter 5-speed Sturmey-Archer SW super-wide-ratio 3-speed Sturmey-Archer T,TB,TBC,TBF,TBFC,TF wide-ratio 2-speed Sturmey-Archer TC close-ratio 2-speed Sturmey-Archer TCW wide-ratio 3-speed w/coaster