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Effective Gear Ratio

Discussion in 'Lifted & Offroad Suspension' started by cbroberts88, Nov 28, 2012.

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  1. MrShorty

    MrShorty Moderator Staff Member 1000 Posts 100 Posts

    The thing that helped me get it straight in my head is to think in terms of torque/force rather than "effective gear ratio." The math looks like this (starting with an arbitrary input torque t0 at the driveshaft):

    ta=t0*gr: ta is torque on axleshaft, gr is gear ratio
    F=ta/r: r is tire radius (=1/2 diameter) F is Force between wheel and ground (this is what makes you go).
    combined: F=t0*gr/r
    solve for t0: t0=F*r/gr
    for comparing when something changes:
    t0=F1*r1/gr1=F2*r2/gr2 (remember that t0 doesn't change for our purposes)
    if we assume 1 is the "stock" configuration, then to compare 2 to 1
    F2/F1=(r1/gr1)/(r2/gr2)=(r1/gr1)*(gr2/r2)
    Example: stock gear ratio 3.55 with a 30 inch diameter tire
    new is 3.73 with 35 inch diameter tire
    F2/F1=(15/3.55)*(3.73/17.5)=.90 so the new configuration will put 10% less torque (F2=90%*F1) to the ground and will thus feel more sluggish than stock.
  2. the phantom

    the phantom Active Member 1000 Posts 100 Posts

    Also some things the math does not take into consideration is if your lifting your truck you will have more wind resistance and also if your adding any extra weight from larger tires and rims. It might not be much but it is a factor. So my recomendation is to always round up to the next available gears or even the next after that. Just my .02.
  3. Crawdaddy

    Crawdaddy Moderator Staff Member Platinum Contributor 1000 Posts 100 Posts

    Moved to the Lifted and Offroad Suspensions sub-forum and stuck. :great:
  4. cbroberts88

    cbroberts88 New Member

    At first this hurt my brain but once I broke everything down it became very simple. Great formula!


    Thanks!
  5. K15 Blazer Guy

    K15 Blazer Guy New Member 100 Posts

    wait, im confused... so...

    (33 / 29) x 3.55 = 4.03

    to run 33" tires, i should have at lease 3.83 gears for the axles?

    i have 265/75 (about 29") and i want to go to 285/75 (about 33") .... i still dont know what gears i should have...
  6. dsfloyd

    dsfloyd New Member 1000 Posts 100 Posts

    so with the formula you used at a pure conversion to get near a stock gear ratio you would need 4.03 gears. I dont believe they make those so after that it is based on your needs. you have a 4.10 or a 3.73 I believe as options. A 4.10 is going to get you close to the stock ratio but a tiny bit shorter ratio which will give you a little more ability to get the blazer moving.

    look at it this way as well
    (29/33) x 4.10 = 3.608 effective gear ratio so real close to 3.55
    (29/33) x 3.73 = 3.28 effective gear ratio so taller than stock by a decent margin and a little more effort required to get things rolling.
  7. K15 Blazer Guy

    K15 Blazer Guy New Member 100 Posts

    ooooo-k cool thanks! =)

    im gonna stick with my 265/75R16s

    31" looks, and handles well with my 3.73 axle
  8. K15 Blazer Guy

    K15 Blazer Guy New Member 100 Posts

    although, 32 would almost be 3.55.... just a little under!

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