Which speedo gear?

Discussion in 'General Ranchero Help' started by MarkB, Aug 24, 2024.

  1. MarkB

    MarkB In Second Gear

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    Nipomo, Ca
    I have a 65 Ranchero, 289, C4, stock rear end (2.80?), 205/70 14's. Of course the speedometer is off, but I'm confused as to what gear I need to change to. Can anyone shed any light on this for me? Thanks!
     
  2. beerbelly

    beerbelly In Maximum Overdrive SILVER MEMBER

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    Portland OR
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  3. Basstrix

    Basstrix In Maximum Overdrive BRONZE MEMBER

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    I plugged my numbers into the calculator and it comes up with 19.5 teeth (rounded up to 20), but I know a 21 tooth is accurate for both speed and odometer. This calc uses the unloaded rear tire radius based on tire size. My tires are 255/60/15 and has an unloaded radius of 13.5". The loaded radius is the correct value to use for this sort of calculation. My 255/60/15's measure 12.7". I was curious so I checked what gear would be calculated for a 255/50/15. The calc came up with a 21 tooth. The unloaded radius for the 50 series tire is 12.5".

    If you decide to buy more than one gear, I'd go with more teeth, not less, based on the above.

    I can help you dial in which gear you need, but you'll have to take a some measurements. The first is an accurate measure of the rear tire loaded radius. The second is the speedometer reading when you're going 60 mph using a GPS. You can get a phone app that has GPS speed. Speedo value when GPS reads 60 is needed. It would also be helpful to know how many teeth your current speedo gear has. All 3 of the above will assure an accurate recommendation.

    If you want to do it yourself, here's the formula:

    # Speedo Teeth = ((5280/(2*LoadedTireRadius*3.14)/12)*((RearGearRatio*DriveGearTeeth)/1000))

    The number of teeth on the drive gear may be hard to come by unless you remove the tailshaft housing and count them.
     
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