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Thread: Automatic Tranny RPMs?

  1. #1
    Z Club of San Diego Montezuma's Avatar
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    Default Automatic Tranny RPMs?

    For you Automatic Tranny drivers, what are your RPMs at 80 MPH? Mine is at 4000 and I am not sure if that is good. 80 MPH? You might ask; well I live in Southern California so 70-80 is the usual speed.....and "No" I don't plan on replacing the the Auto with a 5sp....if I want to play shifter games I can drive my G35 MT6 Coupe or my rough looking 78Z.

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    Semi-retired admin Arne's Avatar
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    Depending on the year and axle ratio, that's in the right ballpark. With the 3.54:1 ratio that was stock for the early automatics, the speedometer should read approximately 19 MPH per 1000 RPM. That doesn't factor in tire height, but tire height only affects true speed, not the relationship of the speedo to the tach.
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    Registered User beezee's Avatar
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    My 74 260 runs 4000 rpm @80 mph 3.54:1 gear

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    1978 280Z (stock) TomoHawk's Avatar
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    My 78 is about the same as Montezuma's. My tires are a little larger too.

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    Former frequent poster sblake01's Avatar
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    Quote Originally Posted by TomoHawk
    My 78 is about the same as Montezuma's. My tires are a little larger too.
    A little larger than what? Unless I missed it no one mentioned an actual tire size.
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    1978 280Z (stock) TomoHawk's Avatar
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    Mine are 215/70/14s. Just slightly larger than stock spec.

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    beandip beandip's Avatar
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    the third gear in a auto is 1 to 1 the same as forth gear on a 5 speed . According to the calculator for tires at the tire rack. I am running 3.70 ratio and am turning 3K at about 60 mph My speedo is not accurate and this info is being clocked by a pace car . Gary
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    1978 280Z (stock) TomoHawk's Avatar
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    I only know at low speeds, when approaching the mobile radar-speed sign, that my speedo is correct- at least up to 50 MPH on a certain 2-lane blacktop road nearby.

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    Z Club of San Diego Montezuma's Avatar
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    I am running 195/70/14s, on stock rims.

  10. #10
    1978 280Z (stock) TomoHawk's Avatar
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    Don't forget, the RPMs you turn at any speed also depends on the efficiency of the driveline. A torque converter that slips will give you more RPM.

    So use a higher stall speed TC?
    Last edited by TomoHawk; 09-22-2006 at 05:12 AM.

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    Former frequent poster sblake01's Avatar
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    Your torque converter should be fully locked up at those speeds. If it's slipping then you have other problems. I think you're confusing the meaning of 'driveline efficiency' which has to do with how much torque/hp at the flywheel/flexplate actually gets to the rear wheels.
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    1978 280Z (stock) TomoHawk's Avatar
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    No, I'm talking about how many RPM get to the rear wheel, which depends very much on how much the torque converter slips; and they do slip some at all speeds. Otherwise, you should be getting all the RPM from the engine to the wheels. Driveline drag only makes the engine turn harder to get the car moving.

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    Former frequent poster sblake01's Avatar
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    Whatever you say. But the converter doesn't slip beyond what it takes to make gear changes, you know kind of like the action of a clutch with a manual tranny. It's gearing, both transmission and differential, that determines how many rpm get to the rear wheels. Now, please, no more on this. The question was 'what are your RPMs at 80 MPH?'
    Last edited by sblake01; 09-22-2006 at 06:30 AM.
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    Registered User beezee's Avatar
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    Quote Originally Posted by Montezuma
    I am running 195/70/14s, on stock rims.
    Just checked ,same tire and rims.Our drivelines appear the same including our tach and speedo readings

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