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Gear Question fur da Brainy-ee-acks

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Big10, Jan 7, 2003.

  1. Big10

    Big10 Registered Member

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    Okay first...If I have stock gears (3.08) which i do, and thirty five inch tires, which i do, how many RPM's should I be turning at 55mph? I would assume that this figure would be low, considering the tall gears and oversized tires. However, I turn around 2100 Rpm's at 55mph. Could that be right? Or....is there a possibilty that the TC has been modified by a previous owner? Also, it has fairly good "ass-in-the-seat" power, and It shifts into 3rd gear around 35-40 mph.

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  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I'd guess you have 3.73's. Pull the covers and see for yourself, it's prolly a good time for some diff maintenance anyways.

    rene
     
  3. imiceman44

    imiceman44 1 ton status

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    I would say the same, check your gears yourself, I think it should be closer to 1800 if it's 308.
    I have 3.54 with 35" tires and it shifts into third at 35, it doesn't mean anything.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    You need to figure out how fast you are actually going. Your speedo is WAY off, of course, given that your tires are a true 35" and your gears are really 3.08's, you'd be pulling 70MPH with a 1:1 tranny ratio.

    Rene's thought is backed up by the numbers, change 3.08 to 3.73 in the MPH formula, and you end up with 58.6MPH, which is believable, especially since tire size is rarely as advertised.
     
  5. Big10

    Big10 Registered Member

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    I pulled the covers to do maintenance about a month ago. I checked the R&P, and plainly stamped on the ring is 43:14 (3.07) (i counted them also) Heres something weird too...my speedo is off about 3-7 mph, but its not off on the low side, it actually reads faster than I'm running...like I had really low gears or something. /forums/images/graemlins/confused.gif
     
  6. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Well if you have 3.07(8)'s for sure then at 55 you're turning 1720 rpm with a true 35" tire. There is no mod to the t-case that'll change this. If you're turning 2100 either your tranny is slipping or the speedo is way off.

    Rene
     
  7. Big10

    Big10 Registered Member

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    hhmm...reckon i need to look alittle further into it..I think my tranny is fine. I'll check true tire dia. and do another speed check.


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  8. imiceman44

    imiceman44 1 ton status

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    Do you have any conversion block on the tcase output or is the speedo cable directly mounted to the case?
    If you have one that was inetended for a different size you might have a speedo that goes faster.
     
  9. Big10

    Big10 Registered Member

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    speedo mounts directly to Tcase.
     
  10. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Find one of those speedo test sections on the freeway. Make note of your exact odometer reading at the beginning and at the end and you'll know exactly how far off your speedo is. It sounds like your speedo is about 20% slow to me...which makes sense because your 35's are about 20% bigger than stock too.

    rene
     

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