Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

Gear ratio / speedo error

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by Squishy, Mar 5, 2003.

  1. Squishy

    Squishy Registered Member

    Joined:
    Aug 28, 2002
    Posts:
    5
    Likes Received:
    0
    Location:
    Dublin, California
    We have 38s on our Blazer and the rear ratio is 4.10:1. Does anyone know if it will be necessary to put an adapter on the speedo cable to allow for the error? There is no cable installed now, and it spins about 2100rpm at 55-60mph as far as I can tell. I have an adjustment unit to go between the tranny and speedo cable, but I'm just wondering if it will be necessary.
     
  2. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

    Joined:
    Mar 5, 2001
    Posts:
    15,684
    Likes Received:
    1,392
    Location:
    642 Days to BB2018
    I've got a little "Stewart Warner" offset adapter that was installed between the speedo cable and the xfer case speedo input. It looks like it was put there to re-gear and correct for larger tires.... Maybe there's a way to find one like that?

    I've also heard that speedo shops that "calibrate" speedos DON'T actually change gears the way everyone thinks....they actually have a small speedo setup with a strong electromagnet and they somehow apply magnetic field to recalibrate your speedo to match the "reference" speedo they use..... it's weird sounding, but I've heard it from a few different people "in the know". I'd like to see that in person someday. /forums/images/graemlins/cool.gif
     
  3. Squishy

    Squishy Registered Member

    Joined:
    Aug 28, 2002
    Posts:
    5
    Likes Received:
    0
    Location:
    Dublin, California
    Yeah, I've got one of those adapters too. I guess I could just install the cable and road test it some and then work out the numbers.
     
  4. 70~K5

    70~K5 1/2 ton status

    Joined:
    Apr 13, 2002
    Posts:
    2,032
    Likes Received:
    0
    Location:
    sillycon valley
    You can change the gears inside the adapter to match up your setup. /forums/images/graemlins/thumb.gif
     
  5. Squishy

    Squishy Registered Member

    Joined:
    Aug 28, 2002
    Posts:
    5
    Likes Received:
    0
    Location:
    Dublin, California
    I didn't know the adapter could be changed. Thanks for the tip! /forums/images/graemlins/shocked.gif
     
  6. 70~K5

    70~K5 1/2 ton status

    Joined:
    Apr 13, 2002
    Posts:
    2,032
    Likes Received:
    0
    Location:
    sillycon valley
    [ QUOTE ]
    I didn't know the adapter could be changed. Thanks for the tip! /forums/images/graemlins/shocked.gif

    [/ QUOTE ]
    Well some are sealed and some can be opened to adjust the ratio to your truck. Depends on which one you have. /forums/images/graemlins/thumb.gif
     
  7. MudNurI

    MudNurI 1/2 ton status

    Joined:
    Jul 15, 2001
    Posts:
    2,113
    Likes Received:
    0
    Location:
    VT
    hmmm I had a local cop clock me at 25, 40, 50, and 65 in my truck (readings from my speedo) to find out how far off I was and if there was a difference at each speed- my average is 9 mph off.

    This was after messing around with the parts counter at a local Chevy dealer for over an hour while they tried to figure out how to fix it....

    Brandy
     
  8. Steve_Chin

    Steve_Chin 1/2 ton status

    Joined:
    Mar 6, 2001
    Posts:
    1,126
    Likes Received:
    0
    Location:
    Foster City, CA, USA
    The correct way for a speedo shop to calibrate a speedo is to ensure that the odometer reads correct at 1000 revolutions per mile, then make the speedometer head agree with that by either moving an adjuster in the speedometer head or by changing the magnetism of the flying magnets in the head.

    That said, you first need to figure out what gears you have in your transfer case (you already know your tire OD and final drive ratio). To calculate what gears you need, you first need to know the Speedometer Gear Ratio (SGR).

    Calculate that by:

    FDR = Final Drive Ratio
    TOD = Tire Outside Diameter

    SGR = 63360*FDR/(TOD*3141.597)

    In this case, with 38" tires and a 4.10:1 final drive, the SGR = 2.176030384

    Next, you need to know the tooth counts on the drive gear and driven gear. The only way to find out is to remove them and count.

    If you divide the number of teeth on the driven (output) gear by the number of teeth on the drive gear (the gear on the output shaft) and that result does not agree somewhat closely with the SGR, you need a ratio adapter (or you need to change one or both of the gears).

    In order to determine the ratio of the drive adapter you need, you simply:

    SOG = Speedometer Output (driven) Gear
    SIG = Speedometer Input Gear
    SGR = Speedometer Gear Ratio

    Adapter Ratio = SOG/SIG/SGR

    Note that some adapter ratios might be overdrive (greater than one) and some might be underdrive (less than one).

    Clear as mud?
     
  9. MudNurI

    MudNurI 1/2 ton status

    Joined:
    Jul 15, 2001
    Posts:
    2,113
    Likes Received:
    0
    Location:
    VT
    boy after reading that- I'm glad I got a cop buddy who was willing to run radar while I messed around...

    Brandy
     
  10. Squishy

    Squishy Registered Member

    Joined:
    Aug 28, 2002
    Posts:
    5
    Likes Received:
    0
    Location:
    Dublin, California
    Clear as mud, definitely. I appreciate the info! One question, though - don't we have to take into consideration the ratio of high gear in the tranny as well as the diff in order to find the 'final drive' ratio? Just curious, I've read some conflicting info on this subject.
     
  11. gallup

    gallup Registered Member

    Joined:
    Sep 24, 2002
    Posts:
    8
    Likes Received:
    0
    Location:
    Ozark Booneys MO
    Ok, IF we can compute what gears/adaptor to use on our rig to compensate. Where can you buy such? Does the GM dealer handle the adaptor gears and the ad on adaptor? My local GM dealer is not interested in helping with this. Any of you have a good parts counter at your local dealer? Let me know there name and I will call them!
    thanks,
    Mike
     
  12. Roostr84

    Roostr84 1/2 ton status

    Joined:
    Aug 19, 2000
    Posts:
    451
    Likes Received:
    0
    Location:
    NJ
    I think that in Steve's explanation the "final drive ratio" is the high gear and the rear end gear combined, i.e. 1:1 final tranny gear by 4.10:1 gear ratio in the rear.
     
  13. Steve_Chin

    Steve_Chin 1/2 ton status

    Joined:
    Mar 6, 2001
    Posts:
    1,126
    Likes Received:
    0
    Location:
    Foster City, CA, USA
    Tranny gear doesn't matter, since the speedometer gears are downstream of the transmission, in the output of the transfer case (this would also be true of a 2WD unless the 2WD had an overdrive/underdrive box mounted aft of the transmission output). The reason I'm calling it "Final Drive Ratio" is because we hae rear ends and front ends in out 4x4s, so calling it a "rear end ratio" would just be plain silly.

    Oh, and 63360 is a constant: It's the number of inches in a mile. 3141.597 is a product of two constants: Pi (a close approximation of pi, anyhow) times 1000 (the number of revolutions per mile the speedometer/odometer are supposed to see).
     
  14. Steve_Chin

    Steve_Chin 1/2 ton status

    Joined:
    Mar 6, 2001
    Posts:
    1,126
    Likes Received:
    0
    Location:
    Foster City, CA, USA
    The gears and adapters should still be available from GM. Unfortunately, I don't know the part numbers for the gears. I do know that the ratio adapters are available from GM (I bought one with a 1.2020:1 ratio recently) and the part numbers don't make sense. I can tell you that the adapters are from group 4329. The part number of the 1.2020:1 adapter is 015618397 and GM list on it is $61.18. If you can obtain the correct gears for your application, you'll get off cheaper, but some tire/final drive combinations won't allow that.

    Oh, I have found that the guys at Fairway Chevy in Las Vegas are extremely helpful. 800-395-1440 or 702-641-1436.
     
  15. ZonkRat

    ZonkRat 1/2 ton status

    Joined:
    Jan 16, 2003
    Posts:
    1,003
    Likes Received:
    0
    Location:
    Lawrence County Tennessee
    This doesn't help much toward fix,but,I check the speedo reading against a handheld GPS over average of about 5 miles.Works well on long straights too. /forums/images/graemlins/thumb.gif
     

Share This Page