# Overdrive?



## Omni (Jul 7, 2016)

Good Evening All

It's been a while since I've posted; been working on a couple of vintage lawn tractors. Now that we are in the depths of winter, I've turned my attention back to the cars.

I would like to slow down the rpm's of the wife's '66 GTO tri-power 4 speed. The car is stock down to the 14" wheels and the 3:55 posi. Modifying the sheet metal is not an option. 

I'm looking at either replacing the 3:55 with a 3:08 or a 4 speed overdrive if one is available that will bolt up to the stock bell housing.

What about a stock 3 speed overdrive from the late '50"s?

As always, all thoughts and comments are welcomed.

Omni


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## PontiacJim (Dec 29, 2012)

Omni said:


> Good Evening All
> 
> It's been a while since I've posted; been working on a couple of vintage lawn tractors. Now that we are in the depths of winter, I've turned my attention back to the cars.
> 
> ...


3 speed OD from the late '50's would probably not hold the torque and no doubt fitment issued would be a real problem.

A 4-speed with OD is called a 5-speed. Several out there like TKO or Richmond. The TKO requires the floor to be cut and modded due to it's size and I don't think the Richmond does - but check into that. The TKO uses 5th gear as an overdrive and you have to go up with the rear gears to around a 3.70 - 3.90 range to really take advantage of it. The Richmond on the other hand is a 1:1 5th gear, but has lower gearing 1-4 so you would need to drop the rear gear ratio down to something like 2.70's to really get the benefit. Here is a neat comparison: http://www.5speedtransmissions.com/5sp_comparison.html

Not an inexpensive swap.

I would not swap to the 3.08's. Reason being is that it will take more clutch slip to get the car rolling and is a real pain taking off on inclines or hills. It would be an improvement over the 3.55's, and can work, but you'll be using up a lot more clutch disc. You will also lose some performance, of course. 

You did not say if you still have the factory 10.75 compression. The 3.08's will lug the engine down a little more and you may experience detonation/pinging so timing may have to be adjusted. Assume if 10.75, you are already using a good grade of high octane gas.


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