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  1. #11

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    I'm starting to confuse myself thinking about it more and more trying to make sense of it and how to explain it.
    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.
    '98 K1500 Suburban LS 5.7 L 4L60E NV246 ARB
    '92 Ford Explorer XLT 4x4 4.0 L A4LD BW13-54 Trac-loc rear
    "My toys were the greasy cogs and springs and pistons that lay around all over the place, and these, I can promise you, were far more fun to play with than most of the plastic rubbish children are given nowadays." Danny in Roald Dahl's Danny The Champion of the World

  2. #12

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    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.




    2011 Silverado CrewCab 5.3L*Ram Air Look Cowl Induction Reflexxion Hood*6" Pro Comp Lift*Flowmaster Dual exhaust*Perfect Launch Rear Diff. Cover*Led Smoke Taillights*L.E.D.Smoke 3rd Brakelight*60" L.E.D. Tailgate Bar*Rearview Mirror Reverse Camera and Sensors*Smoke Headlight Covers*Front Bumper Grill Insert*Deezee Black Alum. Diamond Rail covers*20" Ultra Motorsports Rims wrapped with 35.5"B.F. Goodrich Tires*Inchannel Raingaurds*CAI*Bullydog GT Tuner*Alpine Amp*Boss 6x9's*Boston Acoustic Subs*Terrantula Tweeters*Custom Sub box*Red Led Interior Accents*5% Ultimacool duo tint rear window and doors, 20% front doors*Custom Vinyl Graphics*Demolded*Debadged*Painted drums and calipers*
    Future Mods Include: Custom Interior Hydrographics, Electronic Rollup Toneau Cover, 4.56 Gears, Painted rear bumper

  3. #13

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    Moved to the Lifted and Offroad Suspensions sub-forum and stuck.
    Christopher

    1991 Chevy Suburban 1/2 ton 2WD w/ chevy SBC 350-3/4 ton drivetrain upgrade w/4.10 gears 200K miles
    2005 Saturn ION-2 Stock 277K miles
    1982 Bronco, 1993 Bronco (sold), 1971 M35A2 Deuce and a Half



    There are 10 kinds of people, those who understand binary, and those who dont...

    Remember kids, the only difference between screwing around and science is writing it down- Adam Savage

  4. #14

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    Quote Originally Posted by MrShorty View Post
    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.
    At first this hurt my brain but once I broke everything down it became very simple. Great formula!


    Moved to the Lifted and Offroad Suspensions sub-forum and stuck.
    Thanks!
    -2008 Chevrolet Avalanche LT 4x4 w/ K&N CAI
    -2012 Kia Soul !
    -2003 Kawasaki Ninja ZX-6R 636

  5. #15

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    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...
    Gas was $1.83 when Obama took office.....

  6. #16

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    Quote Originally Posted by K15 Blazer Guy View Post
    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...
    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.
    David
    2004 Silverado 2500 Crew Cab 4x4, AMSOIL EA air filter, Granatelli MAF sensor, Throttle body spacer, Magnaflow exhaust (true dual to 2 in 1 out muffler), 6" ProComp lift (add a leaf and 5" superlift rear block), Bilstein shocks, 35's (Cooper Disoverer ST) and 4.10 gears, Rhino Liner, EGRUSA fender Flares and widow visors, extended stainless steel brake lines, firestone airbags w/onboard air compressor, Pioneer Avic X940BT navigation, Accel backup camera.
    http://www.gmtruckclub.com/forum/sho...Silverado-2500
    1960 Land Rover Series II 88
    2001 Pontiac Sunfire
    2013 Toyota Avalon Limited (Wife's Car)
    NRA Life Member

  7. #17

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    ooooo-k cool thanks! =)

    im gonna stick with my 265/75R16s

    31" looks, and handles well with my 3.73 axle

  8. #18

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

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