# 1966 GTO gear change



## Omni (Jul 7, 2016)

Good Day All

Have a 1966 GTO 389 tri-power 4speed (Muncie). The car currently has a 355 gear, with 255x14 tires. The rear is a 10 bolt 

The car is used mainly for cruising, no completion driving

I would like to drop down to something like a 308 to slow down the rpm's at cruising speed. 

Any thoughts on this idea.

Also any thoughts on what speedo gears this would need?

Thanks to all who respond.

Omni


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## PontiacJim (Dec 29, 2012)

Omni said:


> Good Day All
> 
> Have a 1966 GTO 389 tri-power 4speed (Muncie). The car currently has a 355 gear, with 255x14 tires. The rear is a 10 bolt
> 
> ...


 Assume the 255 x 14 tire is about 26" tall. It could be shorter if you have wider tires, but lets go with 26" as that was factory size.

3.55's at 70 mph = 3,211 RPM's. Dropping to a 3.08 at 70 mph = 2,786 RPM's or a drop of about 425 RPM's. Not a whole lot, but better. Of course this is for highway cruising. Secondary and back road cruising RPM's would drop accordingly. A good speed/RPM match would be driving your car @ 55 MPH with the 3.08's which will put you at 2,189 RPM's which is a good number in my book. If some wise guy decided to give you a run, a quick throw into 3rd gear at 55 mph would put the engine nicely at 3,191 RPM's giving you an explosive pull going into 4th gear and shutting down your competitor. :thumbsup:

However, there may be a down side. 3.08's are a bit stiff for a 4-speed on the take-off. You may have to use more clutch slipping than you are used to with the 3.55's. I had 3.23's and a HD 3-speed trans (a little lower first gear than the Muncie 4-speed) and went with a taller tire which effectively drops the rear axle ratio and my car acted more like 3.08's. I did have to use a little more clutch slip, especially on an uphill take-off, but it was not a problem. You also want to make sure you have a good pressure plate and disc that will not slip because the gearing is too stiff and the engine's torque overpowers the set-up. Learned that lesson when I was a kid and kept installing NAPA rebuilt clutch kits in my '67 GTO. Got so fed up replacing clutches (and money was also very tight) every couple months that I pulled the close ratio 4-speed and installed an automatic. So keep this in mind. :thumbsup:


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