Cruising RPM

Discussion in 'Towing' started by oldbrown, Jul 25, 2018.

  1. oldbrown

    oldbrown Newbie

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    Good morning. I am curious what RPMS you guys are seeing with vortec 454, and automatic with OD in crew cab dually. Based on GM code, I have 4.10 gears. I have original tire size. My speedo is definitely off. At 2500 RPM in OD it says 70mph but GPS says 63. The 4 variables are gear ratio, tire size, RPM and mph. I have done the math and something does not equate to what I am seeing. I have taken into account the .75:1 ratio of OD in trans in my math but still off. Thanks.
     
  2. Supercharged111

    Supercharged111 I'm Awesome

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    In the dually I get 2000 RPM at 57mph on the cluster which is within 1mph of GPS. I have 225/75/16 tires, 454, 4L80, and 4.10 gears. I forget where I see 2500. In the 1500 with 285/70/17, 4.10, and 4L80 I see 2000 RPM at 62mph.
     
  3. RichLo

    RichLo I'm Awesome

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    I'm usually puttin along at 150mph @ 5000rpm in OD with 4.10, 31" tires. Hope this helps :waytogo:
     
  4. Tavi

    Tavi Vehicle enthusiasts

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    C863A173-7E3F-43DE-840D-634338023E30.jpeg My guess is your gear ratio was altered at some point in its life. Based on the numbers given by each responder and myself in this spreadsheet. 62-63mph puts you at 4.56 gears.
     
  5. oldbrown

    oldbrown Newbie

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    Thanks guys. The 4.56 ratio did come into my mind as well. I think I will physically check the rear end to confirm.
     
  6. Sparkysikes

    Sparkysikes I'm Awesome

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    I just switched gears 4.56 to 3.73.

    AT 65 mph I was arou d 2200
    At 75mph I was around 2600.

    Now 65 is 1800
    And 75 is 2150

    My tires are 215/85/16 rear tires and 265/70/16 Front. Same height but wider
     
  7. Pastor T

    Pastor T Newbie

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    How is it taking off when towing?
     
  8. Lanny

    Lanny OBS Enthusiast

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    If it doesnt end up being your rear gears, it could be your tach reading slightly off.
     
  9. Supercharged111

    Supercharged111 I'm Awesome

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    I just recently flashed an LQ9 swapped RCSB GMT800 and its tach was waaayyy off. I was really surprised, like it was a 6 cylinder tach being fed an 8 cylinder signal. I don't see how that's possible though. Yes the truck was originally a V6 truck but it had an Escalade cluster and PCM.
     
  10. L31MaxExpress

    L31MaxExpress I'm Awesome

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    The LS PCMs have a programmable tach output. In stock form the tach output is equivalent to a 4 cylinder output. My buddies 1997 stock tach is fairly accurate with his LQ4 0411 after I played with the output settings.
     

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