La vie de vélo

Adventures in cycling and life

Calculating Fixed Gear Speeds and RPMs

Just a little math tidbit for determining gear ratios on a fixed gear bike…

If you know RPMs and want to determine speed:

  • SpeedMPH = RPM * FrontTeeth / RearTeeth * WheelDiameterInches * Pi * 60minperhour / 63360inpermile

Which factors out to:

  • SpeedMPH = RPM * FrontTeet / ReadTeeth * WheelDiameterInches * 0.002975

Or if you know speed and want to see how fast your legs will spin:

  • RPM = SpeedMPH / (FrontTeet / ReadTeeth * WheelDiameterInches * 0.002975)

As an example, lets say I have a 48×16 fixie with 26.5 inch diameter wheels. Spinning at 100 RPMs will put me at:

  • SpeedMPH = 100 * 48 / 16 * 26.5 * 0.002975 = 23.65 MPH

And if I’m cruising down a hill at 30 MPH, how fast are my legs going?

  • RPM = 30 / (48 / 16 * 26.5 * 0.002975) = 126.84 RPM
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September 21, 2008 - Posted by | Fixie, Math

1 Comment »

  1. You Got it!!!!!!!!!

    Also to figure out cadence multiply speed by 4.44

    My perfect cadence is 89

    I learned this also around the same time maybe a year or 2 earlier.

    And this works for any kind of bike not just fixed gear!

    And to do 700c tireswhich is actually 622 you add the width of both tires like 25c=50+622=672divided by 25.4cm(one inch) to convert it into inches.

    672/25.4=26.45669291 not quite 26.5 so it is more accurate.

    Comment by nick | September 13, 2012 | Reply


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