Choosing the Right Gear Ratio – Revisited

Q: Choosing the Right Gear Ratio – Revisited

I’m looking for recommendations for the optimum gearing on my differential. I have a 1968 firebird convertible with a 350. Non-stock changes are – TH400, transmission, Rochester Q-jet, headers, P22560 tires and cast iron Pontiac intake. I presently have 2.78 gears and the car is pretty doggy.

I don’t really want to modify the engine or put in a higher stall converter. I am thinking of putting in either 3.23 or 3.55 gears. I do mostly around town driving but occasionally go out on the highway at 60-70 MPH. Does anyone have any suggestions on what is a good ratio?

A: I just went through the same thing with a 2.56 rear and picked 3.23 as a good all around ratio. Should run around 2600-2700 RPM at 60 MPH where a 3.55 would run 2900-3000 or so (ballpark numbers).

Also, keep in mind that if you are simply planning on changing gears that you can’t go any higher than 3.23 without changing the housing, or so I’ve read.

If you are planning a whole rear end swap, you might want to consider using a rear out of a 2nd gen car. The overall width is about 1″ wider, and you will have to have the spring perches cut and re-welded but the 8.5 inch rear and the posi unit are much stronger. Also, these rears are more common and Cheaper than 1st gen rear ends.

FYI…the code PX was used from 1976 to 1981 (or something like that) for a 3.23 Posi.

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