Rear gear ratio question

Discussion in 'transmission and Drive train' started by chromebumpers, Jul 5, 2018.

  1. chromebumpers

    chromebumpers solid fixture here in the forum Staff Member

    Brian:D
    I tried, albeit unsuccessful to find through searches any information regarding the gear ratio that my 68 Catalina with a TH400, 400 cu in and a 2v Rochester carb would have had as OE.

    Currently this car has 15” steel wheels, tires are 225-75-15. I could be wrong but I believe most cars in the ‘60’s came with 14” wheels, what did the Large Pontiacs come with? Tire size translated?
     
  2. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    I remember someone installed a GM 8-1/2"10-bolt 1971-1999 into your Catalina.
    Actually still made but called the GM 8.6" 10-bolt now with 2-cut gears. 2000- present.
     
  3. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    I have to go back & Confirm but I do believe the 1970-1/2 Trans Am was one of the 1st Pony cars with 15" x 7" wheels.

    Buick had 15" inch wheels on their full size cars as early as 1961 or 1963.
     
  4. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    G 78 x 14 was a common tire size back then.

    26.0" tall tires were common.
     
  5. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Fullsize Catalinas could be special ordered with 4.33 gears just like the GTO .

    3.08, 3.23, 3.42. 3.55 gears most common.

    3.90 & 4.33 gears could not be ordered on A/C equipped cars typical.
     
  6. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    Its been several years But I recall you had the rear diff cover off.
    3.42 Posi.
     
  7. chromebumpers

    chromebumpers solid fixture here in the forum Staff Member

    Good memory!
    The new rear is a 3:73. I was just wondering how much of a departure was made between the original ratio and the 3:73?
    At the same time I’m curious what the net adjustment is between the OE tire size and gearing and the current day tire and gearing?
     
  8. Maniacmechanic1

    Maniacmechanic1 solid fixture here in the forum

    No idea what the original gear was.
    Use a GPS Phone Speedometer App.
    Or your GPS Garmin or Tom Tom.
     

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