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Gearing question.... something I always wonder about?

Discussion in 'The Garage' started by ChevyCaGal, Sep 3, 2002.

  1. ChevyCaGal

    ChevyCaGal 3/4 ton status

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    Ok.... gearing question. Say I was to buy a set of 35's, but they measure 33". If I was to gear for them, would I buy gears to keep them at a factory ratio would I buy gears for the 35" tire, or the true 33" they measure at? I see people say I have 35" tires and say well I am going with 4.56 gearing to keep them factory. Well if they are really 33" instead of being factory it's going to give you a bit more power then right? They should go with 4.27's instead which is what facotry is for 33's. Does that make sense? So what does one do.... set up gearing for the size printed on the sidewall, or the true measured size? Just wondering about this because it's been bugging me for quite sometime seeing people gear for their tires size they see on the sidewall and not the true size....
     
  2. mudhog

    mudhog THEGAME Staff Member Super Moderator

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    well the way i look at it is you never know what size you are going to get so go with gearing for the 35's if thats what you plan on buying you might get a true 35 or you might get a true 33 but either way the gears will work /forums/images/icons/smile.gif and theres nothing wrong with being a little lower geared unless you are running to high of rpm's
     
  3. 4X4HIGH

    4X4HIGH 1 ton status Premium Member GMOTM Winner

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    I couldn't have said that any better. I myself would rather have a little more gear than needed for that little extra pulling power.
     
  4. ChevyCaGal

    ChevyCaGal 3/4 ton status

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    This is hypothetical.... I own 38.5's now.... /forums/images/icons/grin.gif

    I'm not asking for people about to buy tires.... I asking if someone owns tires and wants gearing and want to be close to factory ratio... not more power though.....
     
  5. 70~K5

    70~K5 1/2 ton status

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    Go to one of the gearing guides and see what gears you need for your size tires and what RPM you want to run at 65mph with your final ratio in your trans. It depends on what you want the truck to do. You can gear the truck for freeway driving or for trails. But unless you have something like a doubler or a klune etc. you can't have both at the same time. You're going to have to compremise somewhere. Where to do that is your choise.
     
  6. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Steph...there are only so many gear choices available to us. Frankly most tires measure smaller than advertised on the sidewall. I try and calculate my R's using true height...mostly because i ran with no speedo for so long and hate tickets.

    Using my own Jimmy as an example...I plan on 39.5's which are according to Zakk about 38" tall at 17 psi with about 75% tread left. So lets say they're 38.5" brand new at about 22 psi.

    So I have tires that measure a true 38.5" tall. I have a SM465 tranny with no overdrive and the fastest I need to drive is 65 MPH. I have a tach and I know where my motor will comfortably run all day long. 2500-2700 RPM and it'll go all day and return reasonable mileage. If I choose 4.10s my freeway R's are gonna be about 2300 RPM. Close to what I want but likely it'll be abit of a dog around town. If I choose 4.56's my R's will be almost 2600 RPM at 65...and it'll have a little more snap in town. 4.88's put me at 2770 RPM at 65. So my choices are 4.56's which can be had in any military set of diffs (one ton) or buy a separate D60 and pay to regear front and rear to 4.88 to gain slightly more low end. I chose the 4.56's...

    If I had an overdrive that would have changed the whole equation obviously...although with a large tire like a 39.5 I can't imagine using the OD much.

    I have no idea if that helps any but that's how I chose the gears I went with.

    Rene
     
  7. ChevyCaGal

    ChevyCaGal 3/4 ton status

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    Makes sense.... just wondered about that is all... /forums/images/icons/smile.gif
     
  8. HarryH3

    HarryH3 1 ton status Author

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    Steph, it's a moot point anyway unless your friend doesn't drive a Chevy. There isn't a 4.27 axle ratio for our stuff. 4.10 and 4.56 is the choice. With an overdrive tranny I'd go with the 4.56's. Without overdrive, 4.10's would be a better choice.
     
  9. 6.2Blazer

    6.2Blazer 1/2 ton status

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    Ideally you should compare the actual tire sizes of the old versus new to determine the gear ratio, but there are a couple of other considerations.

    Larger tires are typically heavier and have more rolling resistance (especially aggressive mud tires), so even if you change to a lower gear in which the engine rpm is the same for a give speed the vehicle will probably feel a little more sluggish. A slightly lower gear can sometimes overcome this.

    Another big factor is that people overstate the difference in gears and tire sizes, meaning they think that switching to different gears or tire sizes will make more of a difference then they really do. Sure, going from 31's to 44's will make a big difference with the stock gears, or going from 3.07's to 4.88's. But going from 33's to 35's (of comparable type) or from 3.73's to 4.10 gears is really hard to notice by seat of the pants. I've personally had 32, 33, and 35" tires on my Blazer with 3.73 and 4.10 gears. When changing just one step in sizes for tires or gears I honestly couldn't really tell any difference.
     

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