[chuckies76ta]

Here is my opinion on these O/D transmissions. You need to take into consideration your Driving style, Speed limits, Engine rpm which I think is the most important, Rear gear, and Tire size. Where is most of your driving gonna be, on the highway or around your City or Town. I guess if you want the best of both worlds you could move up to the T56 with 6 speed and two O/D's.
I was gonna go with the .72 O/D but that first gear ratio of 3.27 was no good for my setup. A good Friend of mine that is much better than myself with numbers talked me out of it. and he was right. I'm very happy with my current setup.

Anyhow, point is that one size doesn't fit all. Find yourself a rpm,tire calculator online and go from there.

https://purperformance.com/p-29669-rpm-calculator.html

I'm running the new TKX with .81 O/D, 275/60R/15 tire, and 3.73 rear gears. My engine like to cruise at around 2200 rpm. The after market shifter I went with is from Modern Driveline. It uses bellville washers for centering the shifter which I'm very impressed with. https://www.moderndriveline.com/shop...y-rear-socket/

[Formulajones]

Originally Posted by chuckies76ta

I'm running the new TKX with .81 O/D, 275/60R/15 tire, and 3.73 rear gears. My engine like to cruise at around 2200 rpm.

Wow that's only 60 mph....yikes You'd still have to buzz 2700 rpm just to do 75 mph.

[scott70]

Most of my " normal" driving is around town,55 mph and 65 mph. The 2.87 first gear is appealing to me as much as having the overdrive. I will admit I'm not much of a putt putt. The car has its original drivetrain but I put it through its paces. Have been since 1986.

[chuckies76ta]

Originally Posted by scott70

Most of my " normal" driving is around town,55 mph and 65 mph. The 2.87 first gear is appealing to me as much as having the overdrive. I will admit I'm not much of a putt putt. The car has its original drivetrain but I put it through its paces. Have been since 1986.

Just my opinion, but you really need to drive your current setup and find where your engine or combo starts to lug the engine. This will be a starting point of what you'll be comfortable with. After that it will be rear gear change or tire sizes to match up. I was gonna do the .72 O/D, but at 60 mph I'd be at 1900rpm which is not good. My engine starts to lug at around 2000 to 2100 rpm. I can always go back to 3.23 or 3.42 gear if I want more cruising time. So with 3.23 gear at 80 mph would put me at 2500 rpm. but that's not how I want my car to run.

So just for example 60 mph 28" tire 3.23 gear .64 od I'd be at 1500 rpm. My setup would need to go 90 mph just to see 2200 rpm. This is what my friend pointed out to me with the .72 I'd have to drive at 70 mph to keep engine happy.

[Formulajones]

Originally Posted by scott70

Most of my " normal" driving is around town,55 mph and 65 mph. The 2.87 first gear is appealing to me as much as having the overdrive. I will admit I'm not much of a putt putt. The car has its original drivetrain but I put it through its paces. Have been since 1986.

You'll enjoy the increase in first gear. Much more pleasurable to drive around town. That's the benefit of these overdrives. Not only getting a comfy cruising rpm but you also help out the other end of the spectrum at the same time. You get the best of both.

55-65 won't be a problem with your setup in 5th gear with the .68 but if you really don't want to putt-putt then just don't use it at lower speeds if you'd rather hear the engine spinning.

Me personally, 65 mph with this deal is 1800 rpm (3.31/.68/27" tire), I wouldn't consider that putt-putt, that's just nice comfy cruising, no engine noise, and should return exceptional mileage too. Once you get used to the idea of having a muscle car that actually cruises like a new car in overdrive you'll find yourself using it more and more.

It's hard for people to get used to that idea after decades of driving them without an overdrive, they either feel the engine still needs to be spinning higher rpm or they just have an engine that requires some more fine tuning to make it happier at those lower cruise speeds.

You can look at it this way, how slow do you go in 4th gear putting around before you think you need to drop down to 3rd gear? At what rpm do you bring the engine down to before you feel like it's struggling? Well 5th gear will act the same way.

From my experience driving 4 speeds for decades, a typical well tuned muscle car can drive around in 4th gear in 25-35 zones no problem at very low rpm.
Your setup with 3.31's and a 27" tire does 35 mph at 1500 rpm. That should not be an issue at all. Or maybe you like just leaving it in 3rd gear?? The overdrive just gives you another option

[VCho455]

These two calculators may help you decide which way to go. It certainly helped me decide that the 3.27 TKX was right for me.

https://www.blocklayer.com/rpm-gear

https://weddleindustries.com/gear-calculator

[64speed]

I have the TKO600 with the .64 overdrive 3.70 gears and a 26 inch tall tire. The speed limit thru town is 30-35 at which of course I drive in 4th but on the side road home the speed limit is 50. Once I get some speed and RPM up I can shift to 5th at 45 mph and never lug the engine or lose my ability to gain speed. Engine rpm in 5th at 45-50 is just above idle. I feel like my rear gears are a little too deep to be ideal but it makes it really easy to take off and drive 1st is just a little short. When I went to the beach in March I put 22 gallons in the tank and drove 232 miles to the hotel (interstate 90%) and then drove around Myrtle Beach when I got there another 25-30 miles and I still had about 3 gallons. The TKO and the RTX gauges are the best upgrades to the car I have made.

[Formulajones]

The most popular package I've done here is the tko with a .64 and 3.73's.

I've done them with 3.50-ish gears as well. In Fact our daily had 3.55's with a .64 for a few years and it was fine, used it all the time at 35 mph.

I've since put the original 3.73's back in the Z so I could use the 3.55's in another project and quite honestly I can't tell much difference in overdrive. Darn thing got 22 mpg on the last trip with the big fat 780 Holley on it cruising at 75 mph lol.

[scott70]

I’m going to do the 2.87 / .81 OD. With my driving style , 3.31 gear and 255/60-15 tire I think it will be good. It’s about a 500 rpm drop from 4th to 5th which is good enough for me.

[jhein]

Originally Posted by scott70

I’m going to do the 2.87 / .81 OD. With my driving style , 3.31 gear and 255/60-15 tire I think it will be good. It’s about a 500 rpm drop from 4th to 5th which is good enough for me.

That is the exact combo I have. Same trans gears, rear gear and tire. As I recall, you and I both rarely if ever drive at Interstate speeds. 55 mph is really the highest speed limit I encounter. The .81 OD is perfect for 65 and even 70 mph cruising. Even 75 mph @ ~2500 rpm is still very comfortable.

[chuckies76ta]

Originally Posted by Formulajones

Wow that's only 60 mph....yikes You'd still have to buzz 2700 rpm just to do 75 mph.

If we had the interstates like you guys have I would have a T56 installed. I can cruise at 70 mph no issues. And even 80. But that just gets a guy tickets here. Lol. The fastest speed limit here is 110 klms. Like 68 mph. My car would be no good with a .64 od at that speed.

[Formulajones]

Originally Posted by chuckies76ta

If we had the interstates like you guys have I would have a T56 installed. I can cruise at 70 mph no issues. And even 80. But that just gets a guy tickets here. Lol. The fastest speed limit here is 110 klms. Like 68 mph. My car would be no good with a .64 od at that speed.

But at those speeds you're buzzing 2700-2800 rpm, not really helping the MPG or the wear and tear like that. You're pretty much stuck at your 60 mph goal and 2200 rpm.

We have a variety of interstates here that range from 55 to 75 mph. I can use a .64 5th gear on all of them, and even climb the mountain in 5th gear.

There would be no reason at all a .64 wouldn't work in your combo, especially the new .68 since .64's are no longer available anyway.

With your 3.73's and a 28" tire and the .68 overdrive you're only doing 2000 rpm to go 65 mph. If that's the speed you want, 2000 rpm is easily doable for a big pontiac engine. If you have an engine that struggles at those lower rpms then something in the engine setup needs to be revisited.

With a problem like that, you wouldn't even be able to use 4th gear in a 4 speed below 45 mph and that just sounds crazy. I don't know of any muscle car that can't use a 1:1 gear below 45 mph that has 3.73's out back.

Got any driving videos??