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Hot Rods Weird Speedometer question

Discussion in 'The Hokey Ass Message Board' started by blazedogs, Feb 27, 2020.

  1. alchemy
    Joined: Sep 27, 2002
    Posts: 17,101


    TH350's had a wide variety of plastic gears that could be changed in the tailhousing to make the speedo read correctly.
    squirrel likes this.
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 49,560


    most speedometers work at 1000 revolutions per mile. Often a transmission from a car with mild rear gearing is installed in a car with steep rear gearing, and this makes the speedo read wrong. Although the 8" ford usually had mild gears, so that sounds a bit odd.

    Anyways, like he said, there are a lot of gears available for the TH350, and he could probably fix it by replacing the driven, and maybe the drive gear in the transmission. But that takes work....and some knowledge....

    also the first thing to check is if the odometer is reading right, or if it's high, or low. You really want to get the transmission gearing set to make the odometer read correctly, then fix the speedometer if it's still reading wrong. It's pretty common for 60 year old speedometers to read high, low or bounce around, just because they're 60 years old, and worn out, or the lubricant in them is dried out.
    37slantback likes this.
  3. 51504bat
    Joined: May 22, 2010
    Posts: 2,942


    Surveyor's measure in tenths not inches. Or as one said on the stand during testimony in a civil trial in response about how many inches a certain distance was, "only whores and carpenters measure in inches".
    WalkerMD and Ned Ludd like this.
  4. My point is that if a solar wind makes the instruments off enough that that can't be used to plot a road bed or a cemetery plot your speedo is not going to he very damned accurate. I guess if you are not mechanically inclined or are too lazy to do it the traditional way a marginally accurate GPS speedo is good enough.

    Personally I seldom use a speedo in an old car, I own a tach.
    grumpy65 likes this.
  5. Ralphies54
    Joined: Dec 18, 2009
    Posts: 750


    maybe someone's said it already, but GPS doesn't care what your tire size is, it's accurate to the ninth degree.
    Cosmo50 likes this.
  6. trollst
    Joined: Jan 27, 2012
    Posts: 2,066


    Used gps speedo's three times now, love them. They have cop defeating features, you can dial up the last known speed to show the cop when you get pulled over, or, show your buddy how fast he was going when he borrowed the car. Easy to use, set up and cheaper to buy, never have to reset when you change tires.
  7. GPS works by communicating with a satellite or several and determining the distance you have traveled since its last communication. It is the same principle as timing yourself through a specified distance, only you are not required to think.

    What kills me on this whole deal is that the same people who advocate using a GPS speedo will ding me for not having the proper rocker covers. Go figure.
    Just Gary, VANDENPLAS and grumpy65 like this.
  8. I bought a dozen tubes of powdered graphite a few years ago to sell in my swap meet booth. It's used for lubricating speedo cables 95 % of the time, and the other 5% for throttle cables, door locks, and maybe hinges. Nobody seems to get the point of the graphite. Whenever I point it out, all I ever see is a vacant look. To me, a tube of the stuff should be in every toolbox. But people are clueless, even the old-timers.
    It's a tough sell. :cool:
    Tman, Ned Ludd, RICH B and 1 other person like this.
  9. MO54Frank
    Joined: Apr 1, 2019
    Posts: 441


    My off topic 07 Dakota uses the ABS sensor on the driver front wheel for the speedometer input.
  10. olscrounger
    Joined: Feb 23, 2008
    Posts: 4,349


    Lots of ratio adapters to chg the speedo-bolts to trans and cable hooks to it--used many in 40 Coupes with the same setup-350 turbo, 327, 8"
  11. You are supposed to rapping your pipes in a tunnel not be worried about your speed or where you are.
    seb fontana, Turnipseed, Tman and 2 others like this.
  12. In olden times we went to the radio/speedo shop, he hooked up a counter giz to the cable, drove on his measured mile, and then he put together a ratio gear box that went on the tranny; those kinda shops are pretty much gone I think.
  13. The external ratio adapters are still available. There's a guy on the HAMB that assembles and sells them.
  14. grumpy65
    Joined: Dec 19, 2017
    Posts: 925


    I agree with beaner on this one. Know your rpm at relevant speeds and you are good.................................most of the time. Tach has been known to tell me I am doing 100 during a take off from a standing start. Lots of weird smoke off the tires as well. I guess every system has it's glitches...............;)
  15. grumpy65
    Joined: Dec 19, 2017
    Posts: 925


    ^^^ :D:D:D

    (But not having a cable makes packaging in smaller builds a lot easier.)
  16. kabinenroller
    Joined: Jan 26, 2012
    Posts: 690


    Actually there is an old time style speedo shop in Milwaukee. It has been there since horsepower was from real horses! It’s in an old neighborhood, inside an old building full of old gauges. Rodger is the owner and is very knowledgeable about speedometers, cables and gauges.
  17. Last time I felt the need to set up a speedometer I went to the local transmission shop and bought an assortment of speedometer drive gears for the transmission in question. Cost me all of 11 dollars (it was nearly the year 2000). I ran the car between highway markers at 60 MPH according to the speedmeter, timed it and changed the gear. tried it again, took me 3 tries to get the speedometer to within the +/-10% as required for all new cars according to federal mandate. (good information to know on your next speeding ticket). It takes minor intelligence to set up a speedometer (I should know :oops:)

    I had a '73 Olds omega that had a speedometer drive off the left front wheel. It used flying magnets off the hub and ran a cable from the box on the front end. Tire size was no real issue.
  18. Grant Clemens
    Joined: Jan 31, 2020
    Posts: 30

    Grant Clemens

    All old American speedometers or calibrated for 1000 Revolutions of the cable per mile. One only needs to drive or push your car 52.8 feet and the cable should turn 10 times. There are a lot of electronic to cable possibilities It's Only Money.
  19. krylon32
    Joined: Jan 29, 2006
    Posts: 7,419

    Alliance Vendor
    from Nebraska

    We built a deuce 5 window last year and put the GPS pickup module on the back of the interior mirror. Worked fantastic. Once in a while it would take about 1/2 a block to pick up the signal but no big deal. I'm currently doing a deuce 2 door and will do another GPS speedometer.
  20. Arbalett
    Joined: May 12, 2021
    Posts: 1


    I am not very familiar with the mechanisms of speedometers, but I know about the functions of GPS.
  21. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,076

    seb fontana
    from ct

    Hey a new kid. "We gotta steal your club jacket and then the initiation rites have to be set up" :rolleyes:
  22. fleetside66
    Joined: Nov 20, 2006
    Posts: 2,681


    I'm easy & don't get worked up over using non-traditional mechanisms for our hand built machines (disc brakes, fans for our radiators, etc.), but I do think we should try use traditional parts for tradition vehicles when the choice is equal (cost & labor wise). Using a cable driven speedo or tach is not a big deal, so I'd say go with the traditional just might work. My old S/W Wings speedo from the early '50's, coupled with a cable made for next to nothing by another HAMBer to hook up to my 4-speed Saginaw trans is quite accurate. If it wasn't, it really wouldn't bother me, given the fact that is in a hot rod. The GPS is good for my real world people mover, but not for my fantasy world wheels.
  23. Raunchy
    Joined: Apr 16, 2007
    Posts: 378


    I never liked cable speedometers when that was all there was. Having to change gears and add reduction boxes Bouncing needles. I use electric programmable now. It's easy to calibrate when I change quick change gears or tires sizes. Just drive a mile and set it. I love technology.
  24. SS327
    Joined: Sep 11, 2017
    Posts: 338


    Don’t forget about the speedometers that use a permanent magnet generator to generate the signal to the speedometer. Thank you GM!
  25. Legalsers
    Joined: May 13, 2021
    Posts: 2


    I am not very familiar with the mechanisms of speedometers, but I know about the functions of GPS.
  26. Legalsers
    Joined: May 13, 2021
    Posts: 2


    Uses a sort of electromagnetic sensor on the output shaft of the transmission. The sensor generates electrical pulses as the shaft rotates and is connected to the speedometer. The speedometer calculates the number of pulses per mile and converts them into a mile-per-hour reading on the dial. The speedometer is programmable, so any tire size and rear gear ratio works, and it can be reprogrammed if you change gears or tire size. And here not unambiguous. Others use GPS and are not connected to the transmission at all.

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