120 MPH Speedo

Discussion in 'Mustang II Parts' started by fastmonkeywheel, Apr 22, 2008.


  1. fastmonkeywheel

    fastmonkeywheel Member

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  2. Steel Steeds

    Steel Steeds New Member

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    I've got a few, but since you have one already disassembled from the cluster, maybe you could help me out with a question I have. Is there anywhere on that unit, in really small letters, that it tells you how many revolutions (of the cable) it takes to equal one mile?

    I'm changing gears and tires in my '78, and in my research to ensure my speedo reads correctly, most of the calculators require that piece of information. One site said that most "modern" speedometers have that info printed on them somewhere. I doubt our II units would be considered modern, but I thought it was worth checking on, and just haven't had the time to tear one out of a cluster to look. Thanks for any help.
     
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  3. fastmonkeywheel

    fastmonkeywheel Member

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    I don't see anywhere where it has that info. From what I understand switching from say an 85 to a 120 requires no change in the speedo gear. This means that this is a constant and you should just need the rear end and tire size to figure out what speedo gear to get.

    I hope that helps.
     
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  4. Wart

    Wart I'm Mad as HELL and I'M not Gonna Take it ANYMORE! Founding Member

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    I did a bit of web searching and it seems the 'standard' for (most?) American cars is 1000 revolutions per mile to get one mile on the odometer.

    Because the odometer and speedometer are mechanically linked 1000 RPM on the speedometer cable will change the odometer 1 mile in one minute and thus indicate a velocity of 1 mile per 1 minute, AKA 60 miles per hour.

    Since this is a 'standard' both the 120 and 85 mph units should be the same in their requirements for input verses indications, at least in theory.

    I have swapped 120s for 85s in the past and the above held true.
     
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  5. Steel Steeds

    Steel Steeds New Member

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    That is true. I've switched mine out on both cars, and the speed reads true. In fact, on my '77 I had the 120mph speedo tested and it was accurate to within .5 mph!

    I'm changing my rear end gears to 3.80, and putting a T-5 in my '78 King. In order to get the speedo accurate, I have to replace the speedometer drive gear on the transmission output shaft. I ordered a new 6 tooth gear to replace the factory 7 tooth, and I will need to change the factory II 18 tooth driven gear with a 19 tooth unit. According to all of the calculations, and provided that 1000 revolutions per mile is accurate (which it probably is), that will get me as close as possible with the tires I'm planning to run.
     
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