Should I change rear gears or tranny?

Thanks Enjenjo, good to know. I have a 200r4 on order. (Scott) Finally feeling human after my operation, so will have John over next warm day and we will test the diff to see what gears it actually has.


She does also have a clutch fan, 67 never had a shroud so can't use a flex blade.



enjenjo said:
I had a Nova with 3.73 gears and a 28" tire that was just over 3000 rpm at 60 mph. I just crunched the numbers on a 200R4 trans and the rpm would drop to 2300 at 60 mph
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First, it's a real nice car. :) Second, we checked the rear end ratio and it is 3.08:1. So 3500 rpm @ 100 kmh is not right. It's either bad gauges, speedo & tach or it's in 2 nd gear. Would be about 2400 rpm @ 100 kmh as it is in high gear.

Going to a 200 4R trans will drop the rpm to about 1600 so as pointed out by Kermit earlier, the engine with its lumpy cam likely won't like that much. However if adding a posi is the plan then that would be a good time to put lower gears in at the same time. Before doing the rear end I would test drive it with the 200 4R first to see how it runs in OD. To get the engine to run at the same speed it does now would take a 4.60:1 rear end ratio. So it could be that a 3.73 or 3.90 might be a happy medium and will give quite a bit more snap off the line and still run at a lower rpm in OD.

Apex Garage speed calculator

Although it has a clutch fan. I'm not sure it's working properly. After running the engine for a bit and shutting it off it didn't coast at all, and was still stiff to turn by hand. It seems to me that that's not right. So a new fan clutch might lower the noise level and help gas mileage.
 
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Thanks John, it was great to meet you today. Enjoyed the chin wag immensely :)
Definitely have all 3 gears, I can punch it at 100K and she will drop down to 2nd no problem. Ask the wife :) don't really use the speedo, have a GPS mounted so it's in Km. Definitely accurate. Tach possibly, but it is new and reads the same as the older monster type it replaced. checked and it is set to 8.
Confirmed today the 200r4 is getting built, so will give that a go 1st. then we will put a posi in after we see how the OD handles it.
As for the fan, I will call around see if anyone has one in stock tomorrow.

Thanks for catching that too John :) Maybe we should go for a little ride next sunny day??






GFaRT said:
First, it's a real nice car. :) Second, we checked the rear end ratio and it is 3.08:1. So 3500 rpm @ 100 kmh is not right. It's either bad gauges, speedo & tach or it's in 2 nd gear. Would be about 2400 rpm @ 100 kmh as it is in high gear.

Going to a 200 4R trans will drop the rpm to about 1600 so as pointed out by Kermit earlier, the engine with its lumpy cam likely won't like that much. However if adding a posi is the plan then that would be a good time to put lower gears in at the same time. Before doing the rear end I would test drive it with the 200 4R first to see how it runs in OD. To get the engine to run at the same speed it does now would take a 4.60:1 rear end ratio. So it could be that a 3.73 or 3.90 might be a happy medium and will give quite a bit more snap off the line and still run at a lower rpm in OD.

Apex Garage speed calculator

Although it has a clutch fan. I'm not sure it's working properly. After running the engine for a bit and shutting it off it didn't coast at all, and was still stiff to turn by hand. It seems to me that that's not right. So a new fan clutch might lower the noise level and help gas mileage.
Click to expand...
 
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28" tires, 3.08 rear gear and TH350 tranny at 60MPH (100 KM/H) = 2250 RPM

Are you absolutely sure you are running 3500 RPM or are you believing your tach or is it the sound of the engine. Are you sure your tach is set for 8 cyl.

With a 3.08 gear, there is no way you need OD

My last car had a 3.36 rear gear with a T5 and 28" rear tires, I would drop to 1700 RPM at 100 kms. The engine would lug, I hardly ever used OD.

Warren
 
Thanks Warren, I started to wonder about the tach after all this too, so took her into the shop yesterday and had buddy put his fancy machine on her and it is Spot on. So unless there is something screwy with the tranny?

I am Baffled :confused:
 

I run 235.75.15 on my 35 Chevy with 3.54:1 rear axle and a 700R. @ 60mph it cruises at 2k rpm.
My daily runs same sized tyres with 3.73:1 rear axle and cruises close to 2k rpm, give or take.
OD is approx. 30%. Engine RPM needs to be between 1800rpm and 2200rpm at 60-65mph.
 
I took her out yesterday and switched my gps to MPH. She is running 3K at 55 Mph.




MGTSTUMPY said:

I run 235.75.15 on my 35 Chevy with 3.54:1 rear axle and a 700R. @ 60mph it cruises at 2k rpm.
My daily runs same sized tyres with 3.73:1 rear axle and cruises close to 2k rpm, give or take.
OD is approx. 30%. Engine RPM needs to be between 1800rpm and 2200rpm at 60-65mph.
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Good to hear. And 1st gear is a bit lower so a little better off the line too. :)
 
Thanks Guy's :) Yes, 1st gear is much better now too. I was thinking posi next, but then thought it's much cheaper replacing 1 tire than 2... lol:D
 
Late to the game on this one. (Happy Retirement BTW)

But my recommendation would have been putting in the OD trans, either a 200r or the 700r. 200r being almost the same size as a th350, makes for an easier install.

As for the gears in the rears.

Well now that you have the trans, when you decide to upgrade to a posi, just install a set of gears in the 3.2 range and call it a day.

Take a real good look at pricing, options and accessories before you pull the plug on gears, as you may be better off in the end with dropping in a whole new rear end.

I did mine backward in my 57 wagon, rearend with posi first, then I went and dropped a 400+hp LS into it and I have to be careful not to destroy the 11 spline stock axles.....should have just bought the damn 9 inch when I first started!!

With my t-bucket I started with a 350/700r4 combo and 2.88 posi in a jag rear. I could pretty much idle down the 401 at 100k. The next year I swapped the rear for a 3.54 setup, and the car is now super happy cruising all day long down the highway and it doesn't bog out when you put your foot into it.
 
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Hmmm, that's a good idea Keeper. Thanks. Would be nice having disc brakes on the back too, so maybe swapping the whole rear end might be the way to go? :)
 
I would definitely look at the pricing.

For mine I have:

Posi - Gears - Install master kit - $770 USD $100 USD shipping
Then I paid another $300 to have the gears installed and setup correctly.

Disc brake kit was $550 USD, plus shipping, then had to have the axles turned to fit the new rotors.

So all in so far is around $1450USD and I still have 11 spline axles.....

For around $2000USD I could have had a complete bolt in 9 inch rearend with disc brakes.

Thats aftermarket, depending on your car, there are most likely OEM rearends out there that would bolt in for a cheaper price.
 
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