Bike Forums > Converting gear inches to MPH???
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 09-06-08, 01:49 PM #1 MAK Member Thread Starter   Join Date: Jun 2005 Location: Delaware Bikes: Yes, I have bikes. Posts: 1,140 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 62 Post(s) Converting gear inches to MPH??? Is there a chart that takes gear inches, multiplies by stroke count and converts to MPH? I just bought a fixie/SS that has a 48x18 drivetrain. That equates to 72". If I multiply 72 times strokes per minute that gives me inches per minute. Divide that by 12 to make it feet per minute and multiply by 60 to get feet per hour. I then divide by 5280 and that should give me miles per hour. At 80 rpm, my math says 5.45 mph. That can't be right. I know it's got to be faster than that. Where is my math error? For example: 72" x 80 rpm= 5760" per minute 5760"/12"=480' per minute 480' x 60 minutes=28800' per hour 28,800'/5280'=5.4545 mph At 120 rpm it comes to 8.18 mph. To average 15 mph I need to cranking at 220 rpm? I must be missing something. Would someone please educate me.
 09-06-08, 02:03 PM #2 Nikephoros Stratiotika ktemata   Join Date: Aug 2006 Location: Vero Beach, FL Bikes: Posts: 284 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s)
 09-06-08, 02:28 PM #3 StephenH Uber Goober     Join Date: Sep 2007 Location: Dallas area, Texas Bikes: Posts: 11,448 Mentioned: 1 Post(s) Tagged: 0 Thread(s) Quoted: 71 Post(s) Gear Inches One of the three comprehensive systems for numbering the gear values for bicycle gears. It is the equivalent diameter of the drive wheel on a high-wheel bicycle. When chain-drive "safety" bikes came in, the same system was used, multiplying the drive wheel diameter by the sprocket ratio. It is very easy to calculate: the diameter of the drive wheel, times the size of the front sprocket divided by the size of the rear sprocket. (From Sheldon Brown's site) The key being here that it is the diameter, not the circumference of the wheel. So you need a Pi factor in there somewhere. __________________ "be careful this rando stuff is addictive and dan's the 'pusher'."
 09-06-08, 03:34 PM #5 MMACH 5 Cycle Dallas     Join Date: Jun 2005 Location: Land of Gar, TX Bikes: Lucinda--2010 Jamis Aurora Elite & a few others Posts: 3,688 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 77 Post(s) http://sheldonbrown.com/gears/ See how your figures match up to the Gear Calculator results.
 09-06-08, 05:42 PM #6 wmodavis Bill   Join Date: May 2007 Location: HIGHLANDS RANCH, CO Bikes: Specialized Globe Sport, Specialized Stumpjumper FSR Pro Posts: 630 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Sheldon is the guru! I re-iterate go to http://www.sheldonbrown.com/gears/ and digest all links from there and you'll know more then you ever wanted to in about it.
 09-06-08, 06:21 PM #7 UncleStu Senior Member   Join Date: Jan 2008 Location: Far NorthEast Texas Bikes: Trek SU200, old Wards Hawthorne 3-speed Posts: 87 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) A little something extra: For MAK- To convert from cadence(crank RPM) to speed in MPH, with 72 Gear Inches: multiply cadence by 0.2141992. Such as: RPM x 0.2141992 = speed in miles/hr. This is only good for 72 gear inches. For something more generally useful: If you know Gear Inches(GI) and cadence(RPM), multiply by (Pi/1056), which is ~0.0029749. So: GI x RPM x 0.0029749 = speed in miles/hr.
 09-06-08, 09:42 PM #8 MAK Member Thread Starter   Join Date: Jun 2005 Location: Delaware Bikes: Yes, I have bikes. Posts: 1,140 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 62 Post(s) Thank you to all that responded. You were all extremely helpful.