Tire size v.s. engine power

Tires and Wheels (Mopar Hubcaps Too)

  1. willem

    willem Well-Known Member

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    Hi there,

    I am about to buy a set of wheels for my 1972 Coronet Wagon. The tires are a bit large though:

    7J 235/60-15 for the front
    8J 275/60-15 for the back

    I was wondering, the car itself is pretty heavy and with its 225 cui slant six a bit underpowered.

    Could tires like this make the car undriveable?
     
    Last edited: Aug 19, 2019
  2. R413

    R413 Well-Known Member

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    What is your rear end gear ratio? Those are tall tires and they will kill the torque. Could make it a slug that won’t be able to pass or pull a hill very well.

    Remember stating out in 10th gear on your 10 speed bike? no power to get it moving.

    ***With the manual trans it is most likely 3.2 gears which will help.***

    Enter 2 sizes to compare. Use your current size to see the differences.
     
    Last edited: Aug 19, 2019
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    • willem

      willem Well-Known Member

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      Yeah that's what is was afraid of :)

      I dont know what the gear ratio is, but its the stock 3 speed manual at the column
       
    • willem

      willem Well-Known Member

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      Does Tire Width affect the torque as well?
       
    • 451Mopar

      451Mopar Well-Known Member

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      The tire thread width (235 or 275) is just width in mm, so 235mm = 9.25", 275mm = 10.82"
      Sidewall height (aspect ratio) of 60% is 9.25*.6 = 5.55, 10.82*.6 =6.492
      Need to double sidewall height for top and bottom of tire, 11.1 and 12.984
      Add rim diameter of 15 = 26.1" tire height (diameter) for 235/60-15, and 27.984" for the 275/60-15 tires.
      The percent difference in tire size (27.984-26.1)/26.1 = 0.072184, or about 7.2%
      The wider (and taller) tires will reduce gearing about 7.2% compared to the narrower (shorter) tires.
       
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      • PurpleBeeper

        PurpleBeeper FBBO Gold Member FBBO Gold Member

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        no.
         
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        • 451Mopar

          451Mopar Well-Known Member

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          Adding to the earlier post, the 7.2% difference between the shorter and taller tires would be like going from a 3.23:1 rear gear to a 3.00:1 rear gear
           
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          • willem

            willem Well-Known Member

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            thanks for the calculation! allthough i have to admit, i'm not that experienced to know what it's going to do to the car.

            i suppose Coronet becomes turd?
             
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