Formula for engine RPM for rear axle ratio.

Discussion in 'Transmission & Driveline Topics' started by john Gialanella, Nov 5, 2020.

  1. 77 cruiser

    77 cruiser Veteran Member

    Messages:
    587
    Likes Received:
    36
    Joined:
    Nov 17, 2006
    Location:
    Frostbite Falls MN
    3.55
     
  2. jeff swisher

    jeff swisher Veteran Member

    Messages:
    1,835
    Likes Received:
    624
    Joined:
    Apr 26, 2018
    Location:
    Yukon Oklahoma
    Jodi on post 3 was darn near correct with the math and an automatic without a lock up converter.

    336 which is a given to make things work.
    The math for a standard transmission with ZERO clutch slippage is.
    336 X gear ratio X MPH, divided by Tire diameter = RPM

    So you have .
    336 x 3.31 x 80 divided by 26.3 = 3382 RPM
    Now add 10% for converter slippage you get 3720 RPM and you stated you see 3750 at 80 MPH.
    I say about right.

    You could have more or less slippage depending on Drag Coefficient weight and how tight or loose the converter is.
    I have one low stall 12" converter that has 2400 rpm stall but slips over 15% and another converter I have that is 9" and 4500 stall that has about 1% at 7000 rpm.
    The difference is crazy.
     
    77 cruiser likes this.

Share This Page