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Gearing Help

Discussion in 'Differential & Driveline' started by HEEP, Apr 23, 2019.

  1. HEEP

    HEEP Junior Member

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    So, after losing a wheel this last Friday, I have been trying to find parts to repair the brake backing plate. In pulling the axle, I looked at the gears to determine the ratio, as my son stated he can't use 5th gear in the NV4500 on the highway. Just falls on it's face.
    So I see that we have a 2.73 ratio in his K10 which seems really high, and I'm not even sure it came from the factory with that in reading around in some threads here.

    So I am trying to see what we really need as far as ratio to make this truck all around balanced. It is a 1984 K10, 6inch lift with 35inch tires. The trans is as stated, a NV4500 Chevy version, and the engine is a LS 5.3.

    Also, if anyone knows the backing plate part number for this truck, please let me know. Not seeing anything listed anywhere on the web.
     
  2. dvdswan

    dvdswan Full Access Member

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  3. 77 K20

    77 K20 Supporting Member Supporting Member

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    [​IMG]

    A chart like this is often used when researching gear ratios and different tire size from stock. Keep in mind this is for 4th gear, not an OD gear.
     
  4. HEEP

    HEEP Junior Member

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    Thanks for the chart. That helps a lot. The link for the backing plate I believe is for a K20. Bigger plate than the K10 plate from what I can tell on different sites for that part number.

    Bill
     
  5. rpcraft

    rpcraft Full Access Member

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    I've seen some 2.73 front axle gears in 10 bolts. In fact I have one sitting in my garage right now that will be getting converted to 4.10 when all said and done. I think they were typically found under diesel rigs but I am no eggspert on what came in what (gas or diesel). Imagine how much better the diesel rigs would have been if they didn't have shit gear ratio's.

    That being said with a 35 inch tire you are going to be wanting to be in the range of a 4.56 gear with your LS motor and NV4500, otherwise I'd be surprised you can find a use for anything over 3rd gear. Probably burning a shit ton of fuel just because of not being in a decent gear and at peak efficiency...
     
  6. HotRodPC

    HotRodPC Administrator Staff Member

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    Also keep in mind, this doesn't account for converter slip and the 200-400rpm loss of an auto trans if you don't have a model with lock up converter.
     
  7. MikeB

    MikeB Full Access Member

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    Strange chart. It lists 29" tires with 3.55, 3.73 and 3.91 gears as "Balanced power/Gas mileage". Huh??? Power yes, gas mileage absolutely not, even with 3.55. The chart seems to be better suited for smaller engines that need to be turning >2,800 RPM to pull a vehicle down the highway. Might be OK for a pre-69 truck with a 265 or 283, but certainly not for a 350. And probably not a 307 or 305.
     
  8. rpcraft

    rpcraft Full Access Member

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    I doubt you are going to be wanting to spin a straight six gm around 2800 RPM for long.

    I think there is some things that glossed over in the chart that people forget and that it doesn't compensate for OD, and the thing about that is OD is not for optimum power, but rather for optimum cruising efficiency. When you want to be able to haul and maintain your power band you will not be spending a ton of time in OD, so what you can do is go out and test what you have in whatever gear turns into your 1: 1 final drive ratio and see what that nets you as far as engine speed.
     
  9. Marcus

    Marcus Member

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    Okay so I’m putting in a 350 in my GMC. For some reason it has a .308 rear end gear in it. The front is factory .342. I’m going to putting 31x10.5 tires on it and also it’s getting a 700r4 trans. So what gear would be best? Cause that chart and all I’ve read I’m confused because people are saying way too many things.
     
  10. HotRodPC

    HotRodPC Administrator Staff Member

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    If you're going with an OD transmission, especially a 700r4 and you're gearing, I'd go with 4.10's being a 4x4. Lower the better, meaning higher numerically. You'll have to OD to compensate and if you're gears are to tall, it's a bigger strain on the transmisison. You don't need OD with 3.08's. 3.73's would be the tallest I'd even consider with an OD or you won't get to take advantage of the OD so figure 3.73 or better like 4.10's maybe even 4.56 but if you go 4.56 I think you need to swap carriers too so to keep it cheaper I'd consider 4.10's.
     
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  11. Marcus

    Marcus Member

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    Yeah see all that makes no sense to me lol. I mean I know what you mean when you say taller gears but how it all works with the different factors confuses me. I want half decent mpg but also have a decent amount of power all around as well. As balanced as possible. I can’t find any charts or anything that helps me understand it better.
     
  12. 77 K20

    77 K20 Supporting Member Supporting Member

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    Gears and tires work hand in hand- or can work against each other. I find the chart is pretty accurate with your average engine. And the chart says it is for a 1:1 ratio (not taking into account the overdrive). And you aren't supposed to tow in OD anyway...

    If you want something well rounded then try to have the RPM in the green band. You can still tow a average trailer and have somewhat decent gas mileage.

    Don't tow? Live somewhere flat (no passes)? Then look at more of the yellow area.

    Lifted truck, tow large or heavy trailers? Slide in camper that runs your already crappy aerodynamics? Live in a place with hills and passes? Then the red band would be probably best.

    Also a factor is what speed do you typically drive? Interstate speed in this state is 80, but I don't live near an interstate. Most highways here are 65-70. So I just drive 65. Or if you have OD then this then allows easier high speed cruising.

    So figure out what you want out of your truck then go out with a tape measure and measure how tall your tires are. Then cross tire height with the gearing you have. Need more towing/hill/power? Then move to the right on the gearing axis. Or put on smaller tires.
     
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  13. Marcus

    Marcus Member

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    I’m going to be driving anywhere from 35 to 70. Some hills and some flat. May haul a 16ft metal utility trailer with up to 50 small square hay bales or a zero turn mower but nothing real heavy. Will be running 31x105r15 on it. May put a slightly bigger cam and bearing in it but nothing crazy. No lift.
     
  14. legopnuematic

    legopnuematic Supporting Member Supporting Member

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    The old Stovebolt 235's in the big bolt trucks (1.5+ ton) would get run under full load on the governor (3200 rpm) back in the day for hours on end and for the most part held up :)
     
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  15. 77 K20

    77 K20 Supporting Member Supporting Member

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    What type of 350? A "run of the mill" stock 350? I'd recommend what HotRod was saying up above. Possibly 3.73s, but with your OD then I'd say 4.10s. This will give you all the power you'd need when towing in drive and still have a good lower RPM for highway cruising empty.

    https://www.crawlpedia.com/rpm_gear_calculator.htm

    There are also websites like this one if you want to be able to calculate RPM based on vehicle options/specs at different speeds.

    A 31x10.50 tire will be around 30" tall.

    So if you had 3.73 gears running empty at 70 mph then you'd be at 2,047 rpm (provided the TC is locked)
    If you had 4.10 gears running empty at 70 mph then you'd be at 2,250 rpm.
     
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