Speedometer revolutions per mile question

Bitburger
Bitburger Member
in "How To" - Skills, mechanical and other wise

I am attempting to correct a speedometer error on my 49 Coupe. Tires are 205/75/15, rear end is 4.10. I am using a 2004R transmission which has overdrive. My calculations indicate a 10 tooth drive gear and a 31 tooth driven gear is needed but that may be based on 1000 revolutions per mile.

I have read that Hudson Speedometers were calibrated at 2000 revolutions per mile where most other makes and models are at 1000. Any truth to this?

Any help appreciated.

Answers

  • 54coupe
    54coupe Member

    I don't know how many revs per mile any manufacturer used. When I put a 700r trans in Press Kale's '49 convertible, the speedometer was off by 50%. A speedometer cable shop made me a little gearbox that screwed onto the transmission where the cable goes, then the cable screwed into it. They calculated the gearing with information I gave them, which was GPS speed at 10, 20, 30, 40, 50, 60, and 70mph and what the indicated speed was at each of those numbers. The gearbox was approximately golf ball sized, and the factory Hudson speedo cable fit on it without modification.

  • cheyenne7271
    cheyenne7271 Member

    It’s called a ratio box, many manufacturers used them into the ‘70s or so. A google search will bring up places to have one made for your needs. I usually just use the app on my phone and have it displayed while driving until yiu can find a solution.

  • Bitburger
    Bitburger Member

    I was hoping to solve the problem by swapping the drive and driven gears, which I already have, to get the closest possible to the correct MPH. The equation some use includes the revolutions per mile. If Hudson did use 2000 per mile vs what seems to be the industry standard of 1000 per mile the equation and gearing would be considerably different. If that fails then the ratio box could be the solution.

    So, if anyone could confirm what a stock Hudson speedo used that would be great.

  • ratlee2
    ratlee2 Expert Adviser

    Check your speed with a GPS at 55mph and calculate the percentage you are off from what the speedometer reads. Then check the driven gear teeth and swap out with the appropriate gear.

    My Hornet with Hydramatic read 70 at 55mph or 127% of actual speed. The driven gear in the transmission had 18 teeth on it and I used a 21 tooth to correct it which was 127% more teeth to lower what the gauge reads.

  • Bitburger
    Bitburger Member
    https://openforum.hetclub.org/discussion/comment/254301#Comment_254301

    Thanks for the response. I have been swapping out the driven gear without success. Before I do further multiple swaps of the drive gear, which is located inside the transmission, and more driven gear, I'm hoping to get the correct data of revolutions per mile so I can run the calculations before I install more combinations.

    Some speedometers have the revolutions stamped on them. I have 4 Hudson speedos and see no stamp marks.

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