Tach Accuracy

Discussion in 'Troubleshooting & Diagnosis' started by MP81, Aug 22, 2017.

  1. MP81

    MP81 New Member

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    Oct 10, 2016
    Washington Township, MI
    I finally got my car to stop eating speedometer cables (and reconnected it to the back of the gauge after it promptly fell off there the last time, because of course it did), and while driving it around on the freeway (driving to/from Woodward this past weekend), at around 75 mph it said it was turning close to 4,000 RPM.

    Based on my calculation sheet, 75 mph and 4k RPM do not correlate. While I'm not sure the speedo is accurate yet (I believe it was, based on my brother driving behind me on surface streets), I can certainly say the car was not at 4,000 RPM - because that would mean I was doing about 95 mph (and I definitely was not).

    Is this a pretty common thing? I figure the speedo should be pretty accurate given it's an entirely mechanical setup (with new upper and lower cables, and a new, correct, conversion box, along with new correctly-sized tires), so it can't really be wrong. The tach, however, is electrical, so it's entirely possible.

    Thoughts?
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Nov 4, 2004
    Ringgold, GA
    Those old tachometers are notorious for reading high, some not so much as others. What's the transmission/converter combo, rear end ratio, and rear tire size?
     
  3. MP81

    MP81 New Member

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    Oct 10, 2016
    Washington Township, MI
    4-speed ST-10, 3:42 rear end and 225/70R15 tires (27.4" - should be the stock size).

    And that would make sense, given how quickly it gets to "5k" when I drive it. Would make a lot more sense if its reading of 5k was something lower.
     
  4. PalmbchZ28

    PalmbchZ28 Veteran Member Gold Member

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    West Palm Beach Fl
  5. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Ringgold, GA
    Yea, don't have to calculate that to tell the tach is off. If you're good with electrical, there's a thread on here about calibrating a tach, I would have to go search for it. I sent mine to Daniel at Guagemark, he's a site sponsor here.

    Treed on the link, here's Daniel's website; gaugemarks.com
     
  6. MP81

    MP81 New Member

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    Oct 10, 2016
    Washington Township, MI
    Yeah, I'll need to go out and make sure that's what I was seeing, but I'm pretty sure it was 4k, because I was pretty surprised when it was that high (it's not like I'm running 3.73s or 4.11s or something).

    So you just sent the whole gauge (not the cluster/circuit behind it)? I've done small electronic stuff before (replaced many a capacitor on one of my computer monitors, multiple times, and in my mom's TV as well) - not a huge fan, but I can deal with it if it's not too bad.
     
  7. Dave Nelson

    Dave Nelson Veteran Member Gold Member

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    Dec 19, 2003
    Vineland N.J USA
    At 75 mph with a 4 spd you should be around 3150 rpm's.
     
  8. MP81

    MP81 New Member

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    Oct 10, 2016
    Washington Township, MI
    That jives with my calculations - and my fuel consumption. 4k on the freeway would've sucked fuel down like no tomorrow.
     
  9. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Ringgold, GA
    Yes it will, try TH350 with a 4K stall converter and 3.73 gears. :whine: That's why I went with a 700-R4.
     
    Last edited: Aug 23, 2017
  10. Twisted_Metal

    Twisted_Metal Moderator Staff Member Lifetime Gold Member

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    Bloomington, MN
    Does it possibly have a 6 cylinder tach?
     

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