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Speedo Pinion selection with 205 X 15 tires....

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  • Speedo/Tach/Gauges: Speedo Pinion selection with 205 X 15 tires....

    OK..so 6.70 bias tires were roughly 1/2 inch taller than the 205's. My 62' wagon will have a 3sp w/OD and a 3.31 R/A. For the best mph read on the speedometer, would a pinion gear for a 3.73 read more accurately that the 3.31 ??

  • #2
    You did not mention all the specifications of your tires but I'm guessing you have P205/70R15 but if you have P205/75R15 it won't make a difference. P205/70R15 translate to the old 6.40 and 6.70 translates to P205/75R15. If you look at the Shop manual and look at the pinion chart both 6.70 and 6.40 tires use the same 20 tooth pinion for a 3.31.

    The best way is to do a proper time and distance test or try a GPS. Very rarely will the speedometer be accurate at all speeds anyway.

    If this is not what you asked then ask again. Len.

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    • #3
      If your car is drivable and you have a GPS that shows speed, check the speedometer against actual as shown on the GPS. A little math will give you the % of error. Count the teeth on your pinion gear and again do a little math to calculate the closest tooth count for the correction. You should be able to get pretty close, it isn't likely that a 50+ year old speedo is very accurate under the best of conditions, they were more like a "guideline" even when new.

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      • #4
        thanks...and funny both should chime in on speedo inaccuracy....point well taken...

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        • #5
          jackb, the answer to your original question is yes. With my '64 Daytona on P205/75R15 tires it was necessary to increase the pinion count by 2 to match the GPS reading. With a ratio of 3.07, it required a 20 tooth pinion rather than 18 with the automatic transmission. Radial tires have a noticeably different rolling diameter or revs/mile than bias-ply tires of comparable size.

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