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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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4,222 Posts
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
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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