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Regearing and manual transmissions

Discussion in 'Technical Chat' started by kram, Nov 30, 2010.

  1. Nov 30, 2010 at 3:53 PM
    #1
    kram

    kram [OP] Well-Known Member

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    I have a question about regearing and how it relates to a manual transmission.

    From what I understand, when people typically regear, they do so to drive at any given speed with a slightly higher RPM than they were before.

    This leads me to believe that if you were to regear on a manual, the only places you would see a difference is starting out in first gear and finishing in 6th. Is this correct? If you want higher RPM anywhere in between, you could simply delay the shift a little (or gear down) and get a similar result couldn't you? Or am I out to lunch on this?

    It's just that I find it curious that other people find their truck sluggish with 33's and I wonder if it's either because they are driving autos, or perhaps because they are using E load tires when I have D. I admit, when I first put the tires on, they felt a little slower from a standstill, but once the revs are up, it feels prety much the same to me (which is plenty powerful for enthusiastic passing on mountain roads). I do like shifting gears though, and usually drive at higher rpms when I'm feeling the need for speed because I've always owed rev happy cars with either turbos or vtec. I do have to say though, if it wasn't so expensive to regear I would on both my car and my truck so that I could redline in 3rd gear much sooner, rather than beyond the max speed limit around here. Kinda takes the fun out of revving when you blow the speed limit in 2nd gear and you're straight into 5th or 6th.

    Anyone care to comment?
     
  2. Nov 30, 2010 at 4:06 PM
    #2
    08pretaco

    08pretaco Almost there

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    People only regear in order to help the engine and transmission when they go to a larger diameter tire. It helps by reducing the load on the engine by allowing it to cycle more freely.
     
  3. Nov 30, 2010 at 4:09 PM
    #3
    whippersnapper02

    whippersnapper02 Well-Known Member

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    Re gearing affects all gears. Depending on how you go the MPH at a certain RPM will be affected. So lets say you could do 35 MPH at 3500 RPM and you install a set of shorter gears. No you are at 35 MPH at 4500. <----That is not accurate I'm just using it as an example.
     
  4. Nov 30, 2010 at 4:16 PM
    #4
    bjmoose

    bjmoose Bullwinkle J. Moose

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    I see what you're saying but it's more complex than that.

    RPM per speed is still affected in any given gear. Yes, with an manual, you can choose the next lower gear. Typically that's a bigger jump than the adjustment you'd make by a gear change, however.

    So given that at a given speed, with larger tires you're running lower RPM, you're also lower on the HP & torque curve (remember the engine produces increased Torque AND HP as the RPMs increase, until the Peak Torque/HP point is reached, which is typically quite high)

    Given that, both an auto AND a manual will have more trouble holding a given speed in the face of a headwind, a hill, or a need to accelerate. So downshifts will be a necessity more often.

    And, as you've observed, getting started from a standing stop is a key performance metric, especially when wheeling on hills or towing - both typically important use cases for our trucks.
     
  5. Dec 1, 2010 at 6:29 AM
    #5
    Tacoyota

    Tacoyota senile member

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    On my 4 cyl /5 spd manual i can let the clutch out w/o the engine stalling... barely, but idling in 1st on a dirt road isnt going to work since it just rumbles. I'd like to go to 4.33 from 4.1 gears , but dont want to raise my rpm's at 70. so i just hold onto my money. But in that example you can see what the gears can do for you.
     
  6. Dec 1, 2010 at 8:37 AM
    #6
    kram

    kram [OP] Well-Known Member

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    Yeah, I do realize that but I guess I don't really understand how it would affect acceleration once you've left 1st gear since you can change gears at whatever rpm/speed you like. If one driver usually shifts at 2500-3000 rpm (which sounds like the average from other threads discussing that) and finds the truck sluggish and decides to regear, I think they would not find the truck sluggish if the just held the gear longer so they are at a higher rpm at any point through the acceleration curve beyond that speed (which is rather low). Of course if someone was already redlining shifts like it was a WRX, there is no room to grab the extra RPM from. I don't often redline (but do occasionally), but as I mentioned I do rev high and row through gears when I'm in the mood to drive enthusiastically. I find the engine responds very well to this and it sounds awesome when doing so. I'm sure the MPG drops to about zero at the time.

    Of course this doesn't account for the benefits that may be had from the extremes. I'm not into hard 4x4, so I find 4lo 1st gear well and truly low enough for my needs, and where I drive 6th gear was often too tall and 5th too short, so it's actually now a lot better for me to cruise in 5th on hilly mountain roads at 85-90km/h.

    Or maybe I just don't get it at all and I'm completely out to lunch, or just trying to justify my lack of funding to regear my truck :)
     
  7. Dec 1, 2010 at 8:56 AM
    #7
    Pugga

    Pugga Pasti-Dip Free Since 1983

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    Of course you can wind out a gear to get more power but that means you're going faster and off the line you're at low RPM so that doesn't work. Regearing affects your acceleration across all gears. Think of it as a mountain bike and you're pedal power is the engine. You can shift whenever you want but it's easier to get power out of the lower gears but you can't go as fast.
     
  8. Dec 1, 2010 at 9:02 AM
    #8
    AndrewFalk

    AndrewFalk Science!

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    Your RPM's will already be increased as you shift into the next gear, so acceleration will be improved and there will be no need to hold the gear longer.
     
  9. Dec 1, 2010 at 11:39 AM
    #9
    kram

    kram [OP] Well-Known Member

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    Actually, using that analogy... are you faster because you switch from a 34T front ring to a 32T front ring? At any given speed you will have more power compared to the larger ring, but speed is totally variable and so is cadence and choosing when to shift, just like in the truck. This is kind of where the whole question came from for me since I'm a mountain biker and often think about gears. Yes, if you change front rings you will be faster off the line for just a moment and you will change the point where you max out at the other end, but in between it becomes moot on the bike as far as I can tell since you just pick the gear that gives you the cadence range (RPM) that gives you the most power. Seems to me it should on the truck too, once the driver in the regeared truck shifts from first at any less RPM than redline compared to the non regeared truck holding the gear.

    Only if you are both shifting either at redline, or at the exact same velocity. If the regeared driver shifts from 1st at 20 and ends up at 2000rpm (not actual numbers, just an example) and the non regeared driver shifts from 1st at 23 and ends up at 2000rpm, is there any noticeable difference in power at that point? Of course you are right, there is no need to hold the gear longer if you regear but my query is, is there any need to regear if you hold the gear longer? (Other then the changes made to the extremes).

    I'm not trying to argue because I think I'm right. Just wanting to understand it and so far I'm on the fence.
     
  10. Dec 1, 2010 at 11:57 AM
    #10
    Pugga

    Pugga Pasti-Dip Free Since 1983

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    I see where you're going with this but I think you're logic is too simplified. Using the mountain bike analogy, set your front gear at the smallest sprocket, you get moving quickly and have a limited top end. While you're moving you have the set of rear gears to chose from, pick a gear and accelerate as hard as you can, it should be pretty easy. Now to the same thing using the next larger front sprocket (use the same rear gear you chose in the first exercise), it's harder to accelerate. It's all gear reductions. As far as driving habits, you are correct, if you shift at 2,500 RPM, you can continue to shift at 2,500 RPM, but you won't be going as fast with taller gears. You'll also hit your shift points quicker because the range each gear is capable of will be reduced. You will still always choose the gear that is correct for the speed, but you may end up using a higher gear for the same speed after you regear.

     
  11. Dec 1, 2010 at 12:35 PM
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    AndrewFalk

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    I agree with you on this point. Regearing won't really make a huge difference unless your new tires are way bigger than stock...or if you go way overboard with the new gears.
     
  12. Dec 1, 2010 at 2:43 PM
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    Tacoyota

    Tacoyota senile member

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    I wish my 5 spd manual had the ol low gear added to it , just idle along and no regear needed. (low 1,2,3,4,5)
     
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