Correct Speedometer for 73 Valiant

Mopar Interiors

  1. C2ndLTpigeon

    C2ndLTpigeon Mopar or no Car! FABO Gold Member

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    I wanted to see which speedometer is correct for my Valiant. When I purchased the car it had a 120 MPH speedo in it. At higher speeds such as freeway, the speedo would read 85ish but GPS would show I was going 70. The factory differential had 2.73 gears and the correct speedo drive gear. Since then I have an 8.25 rear with 3.55 gears and have the correct 36 tooth red seedo gear. It seems to still have the same amount of offset at higher gears.

    To the point, I recently purchased a parts cluster and it had a 100 MPH cluster in it, I did not know they had another speedo other than the 120 MPH. Could it be that it originally had a 100 MPH cluster or does that not matter since the drive gear determines the speed of the cable and thus the MPH on the gauge. The lower speeds don't look to far off but side by side 85 MPH on the 120 gauge looks pretty close to the 70 MPH position on the 100 gauge.

    157305032_471134374297412_8827297036919504745_n.jpg

    Thoughts?
     
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    • townsend

      townsend Well-Known Member

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      President Nixon signed the maximum speed limit of 55mph in 1974 because of the gasoline crisis.For this reason Chrysler speedometers only went to 100mph starting in 1975. Both speedometers will give the same reading from 0mph to 100mph. Have you installed and tested the 100mph speedometer ?
       
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      • Dartnut

        Dartnut Don't hate me because i'm beautiful

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        100% correct.
        Also, I may add that the speedometer does not have anything to do with how the speed is presented on the dial.
        The speedometer is calibrated by the factory to read say 60 MPH on a 120 speedo and the same on a 100 speedo.
        It is the drive pinion in the tranny that feeds the information to the speedometer.
        The factory recommended drive pinion used is determined by the actual speed of the car if the gearing and rear tire diameter is stock.
        Any change to either will necessitate the use of a speedometer pinion that actually matches the speed of the car as it is presently set up.
         
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        • C2ndLTpigeon

          C2ndLTpigeon Mopar or no Car! FABO Gold Member

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          Then I think the speedometer calibration might be off then. Like I mentioned i have changed to the correct drive pinion gear after the diff change and it was still doing it before with the stock rear. Looks like I should try another speedo and see what it does.
           
        • C2ndLTpigeon

          C2ndLTpigeon Mopar or no Car! FABO Gold Member

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          No not yet but I think I shall and see what happens.
           
        • toolmanmike

          toolmanmike Moderator Staff Member FABO Gold Member

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          Test that 120 mph speedo out and let us know. What size rear tire are you running?
           
        • Murray

          Murray FABO Gold Member FABO Gold Member

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          Don't forget that tire size makes a difference too.
           
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          • toolmanmike

            toolmanmike Moderator Staff Member FABO Gold Member

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            I would much rather change the speedo gear than the speedometer.
             
          • volunteer

            volunteer Well-Known Member

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            There is an exact figure out there, somewhere, that shows the 'default' diameter or circumference of a tire that will maintain your final-drive ratio as it is on your current/actual rear gear ratio. I have used (diameter) of 26.75" as 'default' with very accurate results. It is easier to use circumference as that can be measured with a flexible tape as wrapped around center of tire. I also do both rear tires to ensure very much equal - when rear is a 'limited slip'.
            Convert D to C by multiplying by 22 then dividing by 7 (Pi). So, 26.75 converts to 84 inches. Stock size 205-70-14's may have a D of 25 inches (C = 78.5) so doing the math, a set of 3.23 gears will convert to an actual 3.45 final drive ratio. The speedo pinion would be about a 33 tooth.
             
          • toolmanmike

            toolmanmike Moderator Staff Member FABO Gold Member

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            Again, what size rear tire are you running?
             
          • C2ndLTpigeon

            C2ndLTpigeon Mopar or no Car! FABO Gold Member

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            Running 215 70 R14. 8.25 with 3.55 says red 36 tooth gear.
            Changing a speedo is no biggie for me. I've had that cluster out tons of times.
             
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