Speedometer calibration

Unknown
Unknown
edited November -1 in HUDSON
I have a '52 HH with four speed hydro and 3.07 rearend. The speedometer reads about 20% faster than actual road speed. My first thought was that a prior owner had put a 3.58 rear end in it. I've double checked the rear end first by noting the tag on one of the cover bolts and actually turning one of the wheels and watching how many times the drive shaft turns. Every thing confirms I have the rear end I am supposed to have. I even set up a tach and computed the speed. Got the same answer. I installed a speedometer that had been cleaned, oiled and calibrated a few weeks ago. Same result. I put a second speedometer in and got the same answer. I talked to the HET club guru on transmissions and he told me the only cure was to install a reduction box on the transmission to slow down the chain. I'll do that if I have to but I'd rather do whatever I need to fix it right. Anybody have any suggestions?



George

Comments

  • Unknown
    Unknown
    YEA. Change the driven gear on the speedo cable to the correct one. The one you have now is more than likley from a car with either a 3.78 or 4.10 car. You will have to check further to find the correct # of teeth on the correct gear, but I am sure someone on this site can tell you which gear to use as it is in the Hudson master repair manuel.
  • Unknown
    Unknown
    Thanks, I'll go check the number of teeth on what I have.
  • 53jetman
    53jetman Senior Contributor
    George - according to the master parts book, the pinion gear should have 18 teeth for a 3.07 rear end gear.



    Jerry

    t53jetman
  • Unknown
    Unknown
    Jerry,

    Do you mean the pinion gear for the rear axel or the speedometer driven gear? I think the axle pinion has 14 teeth. This is according to the data on page 11-17 of the Mechanical Procedure Manual.

    I haven't checked the driven gear yet as it's too darn cold to lay on my concrete driveway. I will a little later this afternoon.



    Do you have a part number for the driven gear?



    Thanks,

    George
  • Unknown
    Unknown
    I just checked the number of teeth on my speedo driven gear. It has 18 teeth.



    George
  • SuperDave
    SuperDave Senior Contributor
    george

    The part number for the pinion speedo gear is 307082 The Stewart Warner number is 418845 and as stated above, the number of teeth is 18. That assumes a 7:10 or 7:60 tire size. If you are using a much smaller circumference tire, you would indicate a faster speed than actual road speed. The gear that is inside your transmission is the same for all Hydra Matics regardless of gear ratio in the differential

    That part number is 304657 The Stewart Warner number for that one is DT1315449. I can't imagine that being incorrect unless someone changed the transmission with some other brand car. I don't have a clue what Kaiser or Oldsmobile used, but I doubt it was any different than the Hudson drive gear.

    Hope this helps, Davew
  • Unknown
    Unknown
    Dave,

    My tires are G78-15's. Everything everybody has suggested I check has been okay. The only piece I haven't looked at is the speedo drive gear. As I'm sure you know, it's a pain to get to. I was hoping there would be something easy I had overlooked.

    Thanks for the part numbers.



    George
  • Unknown
    Unknown
    Barring any other suggestions from someone else, and since it seems like you have the correct driven gear for your diff ratio, check your drive gear and if that is correct, the only thing I can hypothesize is something wrong with the speedometer itself. I have seen some, non-hudson, read significantly low or high and had to be rebuilt with new springs and whatever.
  • Unknown
    Unknown
    James,

    Thanks for your comment. I agree with you but I'm going to wait for a little warmer day before I pull out the drive gear. I do all my work in the drive way. In spite of all the tests I've done the symtoms point directly to the wrong rear end ratio. I'm also going to redo all those tests and see if my arithmetic is somehow screwed up.



    Regards,

    George
  • Geoff
    Geoff Senior Contributor
    As you have tried different speedos, it will not be the calibration of the head, but the gear ratio of the drive gears in the transmission. Now what I know about hydramatics I could write on the back of a small postage stamp with a small felt pen, but in the standard and overdrive transmissions, if you want to change the ratio of the speedo drive you have to change both the worm and the pinion, as the size and pitch of the different ratios differs. This is entails removing the rear housing , so if as suggested, you can get an external ratio change gear drive this would be the way to go. My Hornet has one of these little devices on it, and the speedo reads accurately. I have checked the input and output of this device, and it gears the speedo up 10%, so obviously someone has tinkered with it before I got it. I plan on up grading the rear end from 4.55 to 4.1, so I shall remove the auxilary drive at that time.

    Geoff.
  • Unknown
    Unknown
    Geoff,

    Your comments are appreciated. I think if I had a little bit of yours and a little bit of mine everything would come out just fine. It just torques me to add the reduction gadjet when it really shouldn't be necessary. (Not to mention 70 good old Texas dollars to buy it.)

    Regards,

    George
  • Unknown
    Unknown
    George

    I should have read more carefully before jumping in and suggesting another speedometer, I see now that you tried that.
  • Unknown
    Unknown
    Hi, where can a guy get a reduction box? I have the same thing with my 51. Thanks, Will.
  • Unknown
    Unknown
    Runt

    Southside Machine offered them but I think they are out of business. Give me a few and I will hunt down a catalog from another source.

    Best Regards

    James Coats
  • SuperDave
    SuperDave Senior Contributor
    jamcoats wrote:
    Runt

    Southside Machine offered them but I think they are out of business. Give me a few and I will hunt down a catalog from another source.

    Best Regards

    James Coats



    James,

    I am very interested in that product as well. Tried a Google search,but must not have put the right information in. Most of what I find is electronic speedo information. I am replacong a 4:56 with a 3:58. Since the 3:58 is a rear gear used with the Hydra-matic and mine is an OD. The correct combination of transmission speedo gears are not available. An external adapter would make life simple. Sure would look better than grease pencil marks on the speedometer face! LOL.

    Thanks, Dave
  • Unknown
    Unknown
    SuperDave and littlerunt2,

    AC Speedo Electric in Arlington Tx told me they would make just about any reduction you might need. They'll turn it around and make it faster if that's what you need. Their ph is (817) 460 7788. They offered me one to fix my problem but I didn't take because I thought I was I smart enough to find the problem.



    George
  • Unknown
    Unknown
    Second George's motion to check with AC Speedo also try this website:



    http://www.marks4wd.com/products/tyre_match_box/speedo-correction.html



    Because you are running a mechanical speedometer you'll have to select the correct gear up-or-down mechanism from any manufacturer you go with, you'll just have to do the math and figure out which way you need to go and how much, which can be easily calculated by comparing the rear ratio you have, versus the rear ratio your speedo gear is designed for. If you are running a significantly different OD tire, you'll have to take that into account as well, but no big deal. The manufacturer I knew of is no longer in business, but as stated check the source above and also George's suggestions. Electronic speedos are a different animal and boxes for those are available everywhere.
  • Unknown
    Unknown
    Thanks you guys, I'll check them both out next year! Runt
  • mars55
    mars55 Senior Contributor
    Or if you could find one, use a pinion with 23 teeth. This is part number 307058. The Stewart Warner number is 419001. This is a '52 single range Hydramatic with 4.10 gears pinion.
  • SuperDave
    SuperDave Senior Contributor
    More on speedo trivia..Moron Trivia?

    I remembered that I had a speedometer reduction gear in my junk box. I can't recall for sure where I got it, but most likely off an old Ford Econoline I used for parts (69 vintage). I dug it out and found it was mfg. by Stewart warner. It had a part number of 666F X11 on it. As well as a ratio of 1.0500..did a google search and found the following site http://www.garbee.net/~cabell/stewartwarner.htm

    From what I can read, the gears can be changed around for many different ratios. Unfortunitly I am math challanged and haven't yet figured out what ratio I need to correct from the 4.56 gear to the 3.58. Now to find out if these gearsets are still made and avaiable. perhaps this gear box is what the speedo shops are quoting for $125 + ?
  • mars55
    mars55 Senior Contributor
    Here is a web site that calculates speedometer gears for you.



    http://www.bgsoflex.com/speedo1.html
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