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Old 03-01-2021, 11:22 AM
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Default Overdrive gear help

Going with a TKX - ordering soon.

What OD gear is recommended?
0.72 or 0.81

245/60/15 tires in rear
3.42 Gear.
Crower RAIV cam.

I have read too much info and confused myself. Looking for real world experience.

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Old 03-01-2021, 11:38 AM
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I always do the lower OD gear on my Tremics.I would do the .72.Tom

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Old 03-01-2021, 12:23 PM
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The GM version has three OD ratios depending on close or wide ratio. The wide ratio is a 3.27 1st gear which has the 0.72 OD , The close ratio both have 2.87 1st gear but the OD can either be a .68 or .81. With a 3.42 rear gear and a 3.27 1st gear ratio I think 1st gear would be pretty short.

Example 3.27x3.42=11.18 divide this by 2.52 which is the 1st gear ratio of a Muncie M20 and you end up with 11.18/2.52=4.43 effective rear gear in 1st . With the .72 OD at 65 MPH the engine will be at 2278 RPM.

With the 2.87 1st gear ratio 2.87x3.42=9.82 divided by 2.52= 3.89 effective rear ratio. With the same speed of 65 MPH with the .68 OD would be 1912 and the .81 would be 2278 RPM.

Basically the two OD ratios you listed will result in the same RPM so the choice needs to be made what 1st gear ratio to choose. I opted for the 2.87 1st gear and the .68 OD my car has 3.23 rear gear this would put me right around 1870 rpm @65 MPH. My car is not on the road until April so will find out then how it drives.

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Old 03-01-2021, 12:30 PM
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As Tom stated, always go with the lower numerical ratio. As I always say paying that much for an overdrive you want to take advantage of what it offers.

Since you are running a very short 26 inch tire and 3.42 gears you'll want that .68 ratio to keep highway rpm reasonable for less wear and tear, less noise, better mpg. 2000 rpm will only be 66 mph. The pontiac will have no problem at all pulling that around.

I'd do the 2.87 1st and .68 overdrive

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Old 03-01-2021, 12:54 PM
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Thanks guys. I will follow up during the install.
I just installed a McLeod twin disc clutch as well.

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Old 03-01-2021, 06:32 PM
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I would suggest the lowest number. You can always downshift. But if you guess wrong and need more O/D, you'd have to change the transmission. Downshifting is cheaper.

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Old 03-01-2021, 06:43 PM
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Mixing the Crower 60919 cam with overdrive is not a good idea... That cam has too much overlap to be put into highway overdrive duty.. You will keep giving it gas, nothing will happen, then too much will happen then you have to slow it down but it slows down too much.. The cycle continues... I had that cam in a 462 with a Turbo 400 and a 2.56.. It was just plain wrong.. That's a race track only cam.. Look at the Summit 2801, 068, Crower 60918 with 1.65 rockers.. Dual plain intake, Quadrajet...

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Old 03-01-2021, 06:54 PM
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389 has a super point. The OD gear should depend on your cam & torque curve.

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Old 03-01-2021, 09:09 PM
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That Crower 60919 will work just fine with overdrive. If installed on the correct ICL and the engine tuned properly with a good timing and fuel curve , that thing should work perfectly fine. It's only a 230 @ .050 cam way out on a 113 or 114 LSA. In a 455 it's nearly baby butt smooth, and in a 400 it's a mild lope.

Try a 254 @ .050 in a little 302. I'm running the .64 overdrive with 3.55's out back and 27" tires and I can drive that thing around at 40 mph no problem in 5th gear.

If you have a big ole Pontiac with a smaller cam that can't do it, I hate to say it but that's a weak Pontiac in a sorry state of tune.

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Old 03-02-2021, 10:06 AM
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Its a KRE engine with KRE heads. 310 cfm.
400 to a 461 stroker.
520 hp 580 tq.
Tripower. MSD electrics.

Even with my Muncie - she is a monster torque car.
The muncie just died - I drove her home for 1/2 hour in stop and go in 4th gear.

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Old 03-02-2021, 11:15 AM
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Quote:
Originally Posted by 694.1 View Post
389 has a super point. The OD gear should depend on your cam & torque curve.
There's no law that dictates you must always be in overdrive. Simply downshift, or stay in a lower gear, until speed and conditions are suitable to the engine. That also applies to any gear, you wouldnt shift to third when you know it would lug the engine, you'd say in 2nd til conditions warrant it

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Old 03-02-2021, 11:21 AM
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Quote:
Originally Posted by Rugratman View Post
Its a KRE engine with KRE heads. 310 cfm.
400 to a 461 stroker.
520 hp 580 tq.
Tripower. MSD electrics.

Even with my Muncie - she is a monster torque car.
The muncie just died - I drove her home for 1/2 hour in stop and go in 4th gear.
Sounds like a really nice combination. You'll love the overdrive swap. Completely changes the car.

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Old 03-02-2021, 11:34 AM
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Ive got nearly the same combo. kre 310 headed 461. I put a 4.10 gear and went with the .64 TKO 600. You will never wish you had less overdrive so I would go with the lower numerical OD every time. Even with the 4.10 it bombs at 80mph easy. Did drag week with it. 1100 miles.

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Old 03-02-2021, 12:18 PM
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Use the calculator and determine what RPM you will be at when cruising. You want to be at the lowest number/RPM the cam lists as the operating range.

https://spicerparts.com/calculators/...rpm-calculator

A 245/60-15 is roughly 26.5" diameter.

It is possible to go too 'low', contrary to popular belief. You will have a less than desirable driving experience, and potentially less of an MPG result.

TKO-600 ODs are .64 & .82 .

Considering the cam is actually kind of small for the build, you could probably get away with a lower (numerical) OD, such as the .64, but it really won't be much of a difference with an .82.

2500 RPM is a good RPM to use as a baseline, and with your cfg, would be @90 mph with a .64, and @ 70 mph with the .82 .

1st gear with a TKO-600 is 2.87 regardless of which OD you chose, but if you keep the 3.42, keep in mind, at slower speed, you will find yourself skipping gears, and not be able to get into OD with the .64 in some cases, May also find yourself downshifting on steep grade on the highway.

At @ 62 mph, you will be doing 1700 RPM with the .64 OD. So below 60, OD will pretty much be useless.

IMO, it's straight up personal preference when it's that close.

I have 3.25 gears in my 461, it's a slightly lighter car, so the .82 made sense for me.

EDIT: So for me, to cruise between 60 & 75, I will be going 2000-2500 RPM.


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Old 03-02-2021, 12:24 PM
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Oh wow, APT offers 3 OD gear sets, .68, .72, amd .81 .

https://americanpowertrain.com/produ...19-product_cat


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Old 03-02-2021, 01:15 PM
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The difference between a .64 and a .82 is HUGE!! You are talking 15 mph between the two. Instead of cruising at 70 mph at 2000 rpm, you are now pushing the engine to 25-2600 to go 70 mph. That's a 30% increase.

That's not much of an overdrive for the money being spent.

For his setup with the now offered .68 overdrive

1700 rpm will be 56 mph. Pontiac should pull that easily without a need to downshift, but the option to move the lever is always there.

2000 rpm will be 66 mph.

2200 is a nice spot to be on the highway that returns good mpg and less noise.
That'll be 73 mph.


That's about as perfect as it gets.

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Old 03-02-2021, 03:29 PM
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I'm not seeing a .64 OD on a TKO-600. .68 is the lowest I see.

But, to each his own. All I was saying was, the correct way to chose gear ratios is to ensure you are at the bottom of you cam's suggested RPM range at cruise.


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Old 03-02-2021, 06:45 PM
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Quote:
Originally Posted by HWYSTR455 View Post
I'm not seeing a .64 OD on a TKO-600. .68 is the lowest I see.


.
Correct, your first comment referenced the .64

Tremec has now done away with that unfortunately and replaced it with a .68

In my opinion they dropped the ball on that deal. The .64 was never an issue. What they should have done is changed 3rd gear, that's a massive drop.
They call the original TKO600 a close ratio but in reality the only gear that's close is 3-4. It's the same as an M-21 But that' the only shift that is close. 4-5 was never all that bad, nothing near the 2-3 drop at all.
With a 2.87-1.89-1.28-1.0-.64

What's interesting with that is the 2-3 shift is a large .61 ratio drop. The 4-5 is only a .36 drop, nearly half. So doesn't make much sense to me to fiddle with the overdrive ratio and not correct the other gears in the trans. Now 4-5 is only a .32 drop but we still have the main gears spread as much as .98 That's worse than a wide ratio box, and they call this a close ratio. I get a chuckle out of that.

It's why the Pro Motion T-5 looks so much more appealing. Excellent gear ratios that actually drives like a close ratio box, and still retains a .63 overdrive. Basically the same trans that McCloud was offering, and also mimics the LGT700 that is no longer available.

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Old 03-02-2021, 07:54 PM
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Another reason I went with the road race gear set of 2.87, 1.89, 1.28, 1.00, .82 . As long as you don't stay in 1st much, it's fine, and linear.

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Old 03-02-2021, 08:18 PM
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my two cents, i'd rather turn an engine a little too fast than a little too slow.

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