MPH, RPM, GPS discrepancy

Discussion in 'Tech Discussion' started by HD2500, May 29, 2014.

  1. HD2500

    HD2500 Member

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    Location:
    Sparks, NV
    so I've noticed that my speedo reads 2-3 mph faster than what the GPS app says on my phone. doesn't seem to matter if I'm going 45 or 75, always reads 2-3 mph faster. I've checked the GPS app in comparison to a vehicle that i know has an accurate speedo (fire engine), and it reads dead on. So when i think i'm driving 75, actually I'm at like 72-73...
    Well, my Trip meter/odometer read dead on, according to google maps. Give or take a tenth of a mile over the coarse of a 100 mile trip.
    Doesn't the trip/odometer measure off the speedo? weird...
    Also, the rpm that the tach reads is 200-300 rpm faster than what I should be spinning, given my speed.
    example: at 75mph(GPS) it reads ~2500rpm, when in reality i should be closer to 2200rpm. This is going off every calculator i could find on the internet.
    I do run 35x12.5x18's and 4.10's with a 4l80e, and I also have a blackbear tune which adjusted for the larger tires.

    Just kinda wondering if anyone else has discrepancies in their instruments, and what information I should believe.
     
  2. burgundybeast04

    burgundybeast04 Well-Known Member

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    When your truck is turned off do all the needles point directly at zero? I know on my truck the rpm and speed needles are a little above zero and therefore it reads a bit higher than it should. Doesn't bother me since I know it's there and can adjust for it.
     
  3. HD2500

    HD2500 Member

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    Location:
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    They read right at zero, maybe the speedo needle tip is slightly below actually.
    The previous owner had the instrument cluster replaced, due to the classic stepper motor problems. Is it possible it wasn't calibrated correctly?
     
  4. bucket

    bucket Well-Known Member

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    Location:
    West Jefferson OH, Blackstone VA
    That's what I was thinking, the needles just weren't installed perfect when the cluster was repaired.
     
  5. silverado13

    silverado13 Moderator

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    tire size?
     
  6. HD2500

    HD2500 Member

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    Location:
    Sparks, NV
    35" (advertised diameter)
     
  7. kolocar

    kolocar New Member

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    Mar 27, 2020
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    Location:
    Flat Rock, NC
    I have noticed that the speedo on my 2002 Tahoe consistently reads a couple MPH higher than actual speed based on those highway radar speed management signs. Could just be normal thing for Chevys? At 75 MPH (77 MPH on my speedometer), RPMs are about 2200. I have stock wheels and tires (265/70R16) and the 3.73 final drive.