Rear gear ratio question

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?
 
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.
 
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.
 
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.
 
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.

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