Speedo Gear Change Required

[Deleted User]
[Deleted User]
edited January 2013 in HUDSON
Hello all. I just installed an overdrive in a '51 that had a standard three speed and a 4:11 rear. Does anyone know what the right tooth count would be for the speedo to read accurately?

Allan

Comments

  • nick s
    nick s Senior Contributor
    Alan, I spent a couple years looking for the right gear with no luck, however I found that my spedometer is about dead on at highway speeds, though the odometer is off by a couple percent. it's 16 tooth (there is a more common 16 tooth for the later transmissions but it has an opposite handed helical so will not work and even though it dosen't matter(see previous statement) i think the dimensions are wrong as well. so my answer is the speedo may be more accurate with the wrong gear.
  • Park_W
    Park_W Senior Contributor
    edited January 2013
    Allan, was the OD from a '51 or from another year car? That's important because a '51 or earlier OD needs a little reduction box for the speedo cable. '51ODs had to have this 2:1 reduction because they still had speedo drive gears same as '50 and earlier, but the '51 cars' speedo calibration was changed from '50 (Hudson was getting finally getting in step with everyone else, going to speedo reading of 60 mph at 1,000 rpm on the speedo cable vs. the 2,000 rpm used up through '50. So if your OD is '51 or earler, the first step is getting that little box, either an original or a modern substitute. If you have that, or if the OD is post '51, then the proper speedo pinion gear for an OD car with 4.11 axle is 448215, 16 tooth. They're not easy to find these days. Al Saffrahn is one of the first to check with.
  • [Deleted User]
    [Deleted User]
    If your speedometer needle is off and the odometer is nearly correct the magnet is weak in speedometer. Also from what I can tell in the parts book you should have 15 tooth gear for what you have. The chart is somewhat cofusing but thats my guesstimate,
    Roger
  • Park_W
    Park_W Senior Contributor
    Roger, my original '51 parts book shows the 16 tooth pinion for overdrive-equipped cars having 4.11 gears, and 15 tooth for 4.11 cars without OD. No guessing required.
  • [Deleted User]
    [Deleted User]
    I was looking in the 48-54 book - told you it cofuses me . Yes I looked again and I was wrong 16 should be correct. So many variables,
    Roger
  • Park_W
    Park_W Senior Contributor
    edited January 2013
    No argument on that, Roger ... one really has to look closely on that larger chart.
    And please note, my original post is slightly modified (Allan didn't say what year the OD was).

    For any folks who are sending a '50 or earlier speedometer for repair and/or calibration, be sure the shop is aware that Hudson calibrated for 60 mph at 2000 rpm for the cable, not 1000 as everyone else did. Some shops don't know this, and you get back a speedo that's way off in calibration.
  • [Deleted User]
    [Deleted User]
    Yes there's two page's of possible combinations .I assumed He had an "R" unit but at first was not on the overdrive section.
    Roger
  • [Deleted User]
    [Deleted User]
    Thanks all. Park, I realy have no idea what year the overdrive came from, so I'll wait and do the road test. I do have a box if I need one - I had to put one of those on the 38 when I installed overdrive in that one too. Looks like I might get to test on Saturday, so I'll let you know how it turns out.

    Allan
  • [Deleted User]
    [Deleted User]
    A good speedometer shop can make you a custom gear box for your cable to make it all correct if needed,
    Roger
  • Park_W
    Park_W Senior Contributor
    Since it's a single-lever tranny, it's almost certain to be a '51 or earlier. Mine has a late 1950 casting date on the RH side of the main case. If it needs the reduction box, you'll know it right away when the speedo reads twice as high as it should!
  • SuperDave
    SuperDave Senior Contributor
    My '49 speedometer seems to be fairly accurate. But...Since my transmission is sitting on the floor.. I thought I would check out the speedo gear. It's a 4.59 rear, single lever OD with drive master, (cast in 1949). Book says 18 teeth S/W p/n 448296. My gear is 18 teeth but has S/W p/n 448217.. can't find that one anywhere on the chart. I am considering useing a 4.11 that i have in a parts car. We are in very flat country and since the D/M starts in second gear I won't be useing it anyway. Might increase my gas mileage..I'll need a 16 tooth according to the book. S/W 448299 ?
  • Park_W
    Park_W Senior Contributor
    Dave, don't worry a bit about running a 4.11 with overdrive in that car. My '47 C8 with that gearing still accelerates strongly in 3rd overdrive. Your car is a bit heavier, but not that much. I can't find that SW number either. May be from a non-Hudson car.
  • JasonNC
    JasonNC Expert Adviser
    My speedometer is about 10 mph faster than the car is running. I just use my GPS as the speedometer.
  • EssexAdv
    EssexAdv Expert Adviser, Member
    this might be of interest
    http://www.gaugeguys.com/ratiotest.htm

    Lew
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