1. carguy300

    carguy300 New Member

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    Can anyone give me an idea what speedometer gear i would need for my 68 300. reads about 12-15 mph fast. I have 235/70-15's, should have 3.23 gears.
     
  2. Big_John

    Big_John Illegitimi non carborundum FCBO Gold Member

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  3. carguy300

    carguy300 New Member

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    Thanks. Going to check them out.
     
  4. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    Transmission pinion chart is attached. Note that today's radial tires have a lot lower profile and smaller circumference that the tires these cars came with so you probably need to go at least a tire size smaller to correct the speedo error. Click on mail006 to open, not the icon.

    Dave
     

    Attached Files:

  5. carguy300

    carguy300 New Member

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    Thanks for the info. When i bought the car it had 15" cragars. 275/60 back, 235/60 front. Went with magnum style which were available first time for 68.
     
  6. polara71

    polara71 Old Man with a Hat

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    Ive never understood this thought. I understand the premise but My speedometers have never been off by a few MPHs at the most.
     
  7. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    Usually the error will be something in the 5mph range if a comparable radial is used. I suspect that someone may have changed the transmission pinion in the OP's car, or for that matter, the whole transmission to get to a 15mph error. Could also be a failing magnetic coupling in the speed head.

    Dave
     
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  8. carguy300

    carguy300 New Member

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    This car may have came with 14" wheels, I dont know. I dont have any history. my wife followed me to car show, her 70mph, me 82.
     
  9. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

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    Pull the speedo pinion out and see which one you have for starters. If the door post sticker is still on the car, that should give you the factory tire size.

    Dave
     
  10. carguy300

    carguy300 New Member

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    I havnt taken out the gear yet to check it, but the speedometer moves smooth and steady slow or fast exceleration, no jumps or bounching of any kind.
     
  11. carguy300

    carguy300 New Member

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    Unfortunatly the car was painted some time back they never put any stickers are decals back on.
     
  12. FURYGT

    FURYGT Senior Member

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    Determine the height of your current tires and match them up with the height of tires in the speedometer gear chart in a factory parts manual such as the one you can download in one of the previous replies. Cross reference with your rear gear and you should be able to get very close if not exact on the accuracy of your speedometer reading.
     
  13. dart1962_440

    dart1962_440 Member

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    I spent a lot of time dialing in my 300 for an accurate speedometer a few years ago. Got it where the odometer was spot on. Found out though, that the speedometer was off about 7mph (it reads faster that I'm going) even though the odometer is accurate. I left it alone after that as I didn't want to get into adjusting how the needle reads. I figure I'm probably better off going slower than indicated.
     
  14. CBODY67

    CBODY67 Senior Member

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    Unless the car has factory disc brakes, it should have probably had an H78-14 type of tire on it. The P235/70R-15 is very close to the revs/mile as a P225/75R-15 tire, which is just a bit taller than the old H78-15 tire size. For some kind of reference you can find in the FSM for speedometer gears. In this case, it might even be 8.45x15, in the FSM for factory power disc brakes. That should get you closer to where you need to be, I suspect.

    My '67 CE23 with P245/70R-14 (NOS at the time) BFG Advantage T/A radials, the same revs/mile as an 8.55x14 tire, has a speedometer speed reading about 10% fast, BUT the odometer is dead on. Which means the spinning bar magnet in the rear speed cup needs some magnetism adjustment. I did my checks using the mile markers on the Interstates and a digital wrist watch (which dates when I did all of that).

    By comparison, our '66 CL42 is within 2mph at 60 mph, with H78x14 bias-belted BFGs on it.

    When trying to calibrate a speedometer, always get the odometer right FIRST. It's direct-geared to the speedometer cable, as the bar magnet in the rear speed cup of the speedometer head spins the front speed cup, which is attached to the indicator needle. When the odometer is correct, there's a good chance the speed reading will be in specs, too.

    CBODY67
     
  15. 69 300 vert

    69 300 vert Member

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    Verify your speed with a GPS, phone or stand alone. Note the exact difference and calculate the percentage difference. Pull your current gear and count the teeth. Apply the percentage to the tooth count to get the count you need. If the speedo head is good you should be able to get within 2% +/-.

    Robert