[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.

[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

[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??

[chuckies76ta]

Respectfully, I don't really care about MPG. lol At this point in my life I'm just glad I can go to the pumps and get gasoline. With the way the world is I figure governments will find a way to push us out of the hobby. I'm not sure what I have for videos. I'll look into it.
You have some valuable points and maybe my engine just needs more tuning. I don't know. It runs so nice and performs so beautiful I leave it along. Just the maintenance each year. I have KRE heads on it with Dave's road paver. 245/252 I think. It made close to 600 hp on the dyno. 32 deg. I am running a vacuum advance on it. Next time I'm out I will disconnect it see how it acts. It doesn't like 2000 rpm. In O/D it starts to lug in lower rpm. It is a blast to drive for what I'm doing.

[Formulajones]

Originally Posted by chuckies76ta

Respectfully, I don't really care about MPG. lol At this point in my life I'm just glad I can go to the pumps and get gasoline. With the way the world is I figure governments will find a way to push us out of the hobby. I'm not sure what I have for videos. I'll look into it.
You have some valuable points and maybe my engine just needs more tuning. I don't know. It runs so nice and performs so beautiful I leave it along. Just the maintenance each year. I have KRE heads on it with Dave's road paver. 245/252 I think. It made close to 600 hp on the dyno. 32 deg. I am running a vacuum advance on it. Next time I'm out I will disconnect it see how it acts. It doesn't like 2000 rpm. In O/D it starts to lug in lower rpm. It is a blast to drive for what I'm doing.

Really that's all that matters if you're enjoying it as is.

We daily drive ours so MPG is part of the equation. So I tune and tweak things, even on the cars without overdrives, to make them at least somewhat respectable on MPG. It's kind of funny at the pumps, once in a while I'll get the MPG question from someone and they are shocked that these 50 year old cars are doing better than their new SUV's. And that's one of the biggest reasons I look at overdrives. The other is keeping the wear and tear to a minimum, as we know it's not cheap to build these engines anymore either, let alone operate them on a daily basis.

[scott70]

Yeah mine is not a daily driver. I take it out when I'm in the mood or have a minute. I actually googled the tkx thing and found some other forum opinions...found some were asking about tkx overdrive in the same configuration as mine ,gears,tires,etc......68 vs .81.....opinions were down the middle 50/50 also. Like was said either one is fine...try to figure out your expectations I guess.

[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

[Formulajones]

Originally Posted by scott70

Yeah mine is not a daily driver. I take it out when I'm in the mood or have a minute. I actually googled the tkx thing and found some other forum opinions...found some were asking about tkx overdrive in the same configuration as mine ,gears,tires,etc......68 vs .81.....opinions were down the middle 50/50 also. Like was said either one is fine...try to figure out your expectations I guess.

Yeah you'll get that basically because you have 2 groups of people. Those that struggle to get an engine to run at lower rpm efficiently and those that can.
So you end up with 2 different opinions on overdrive ratios.
Been that way since these swaps became popular 30 years ago.

Usually when I do these swaps I'm also digging into the carb and distributor to tweak and make the engine a little smoother at lower engine speeds.

It's also where electronic control lock up converters have a big advantage in auto overdrives compared to the TV cable mechanical stuff. Not only can I dial in the converter lock precisely but making the engine happier allows me to keep it locked longer.
The only difference here is your left foot is your lock up converter

[scott70]

My 7040267 carb has no port for vac advance. Talked to cliff,,he has added the nipples before on these carbs.

[jhein]

I don't know but I think there's a reason why there are 3 other gears between 1st and 5th. You're supposed to use them. I never liked lugging a motor. I suppose it depends on the motor too. Low compression, small cam motors may tolerate that better.

[chuckies76ta]

^^^^ Ahhh, That's what I'm alluding too.

[chuckies76ta]

Changing the topic a bit, What fluid are you guys running in these TKX transmissions? I ran GM synchromesh in my TKO-600, and I ran the Dexron III in the TKX for awhile then switched back to the GM Synchromesh fluid.

[Scarebird]

Redline D4 ATF.

[Formulajones]

Originally Posted by jhein

I don't know but I think there's a reason why there are 3 other gears between 1st and 5th. You're supposed to use them. I never liked lugging a motor. I suppose it depends on the motor too. Low compression, small cam motors may tolerate that better.

High compression, 254 @ .050 cam and small 302

https://youtu.be/e39e6p7NMY0

[Formulajones]

Originally Posted by chuckies76ta

Changing the topic a bit, What fluid are you guys running in these TKX transmissions? I ran GM synchromesh in my TKO-600, and I ran the Dexron III in the TKX for awhile then switched back to the GM Synchromesh fluid.

I use Amsoil Synchromesh in mine. My sons from G-force they recommended 2 quarts of 85-140 gear oil and a quart of synthetic ATF mixed in.

Thought that was a bit on the odd side but it shifts beautifully.

[M91196]

68 mild 400, 3.55 with close ratio .68 OD TKX with SST shifter mod and 26” tires.
No regrets.
Game changer

[70GS455]

Go .68 OD. If you're concerned with lugging the engine, drive faster before you shift, or just stay in 4th (No different from what you have now) longer. Then you have the extra OD for those times that you need to cruise on the interstate.

If you get the .81, juggling shift points will be of little benefit if you ever need the extra OD.

Recap - with the .68 you can always delay the shift into 5th.

Sent from my SM-T817V using Tapatalk

[jhein]

Originally Posted by Formulajones

High compression, 254 @ .050 cam and small 302

https://youtu.be/e39e6p7NMY0

Nicely done video. But in all fairness, you're not really putting much load on it when you're down low in RPM and trying to come out of it. You're easing into the throttle very gently. I don't really chug around like that so I just downshift.

[chuckies76ta]

Originally Posted by 70GS455

Go .68 OD. If you're concerned with lugging the engine, drive faster before you shift, or just stay in 4th (No different from what you have now) longer. Then you have the extra OD for those times that you need to cruise on the interstate.

If you get the .81, juggling shift points will be of little benefit if you ever need the extra OD.

Recap - with the .68 you can always delay the shift into 5th.

Sent from my SM-T817V using Tapatalk

He doesn't cruise on the interstate. Did you actually read the post from the start.? Did you watch Formulajones video. Most of us don't drive like that..!!!



Here is his post. ( O/P)


(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.)