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Recommended Rear End Ratio - 6 Speed
#1
We've been putting together a '72 fastback 351C 4V project for a few months now. We got the car with no trans installed but included a new in box Tremec T56 Magnum Speed manual. Not my first choice for a trans, but it's a great trans and it was part of the deal so we went for it. So we've spent the last few months converting from auto to manual and finally have it running and driving. 

Now that we have it going it seems pretty clear that it is geared too tall. I can run around my neighborhood in 1st gear or drive at 60 in 3rd . It feels like a car that's meant to cruise at 90 vs get up to 60. 

This isn't going to be a race car, but we want it to be quick off the line and still be able to cruise on the freeway, which I think it is the point of the 6 speed manual. I think the car has a 3.25:1 rear end right now. 

Appreciate any input on rear end gearing to get this thing to be quicker off the line while still being driveable on the freeway.
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#2
Do you know what the overdrive ratios in the transmission are? It looks like 0.63 is the normal 6th gear ratio.

Do you have a more aggressive cam than stock? If so, what are the specs on it, or manufacturer and part number?

What size tires (diameter) do you have?



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#3
I assume it is a 2.66 1st and a .63 OD?? I would go min 3.70 maybe all the way up to 4.11. TREMEC has an app you can download for free that allows you to calculate your speed and rpms with different rear gear ratios.

73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

- Jason


[Image: 082-hot-rod-power-tour-2017-1970-1970s.jpg]
[+] 1 user Likes 73pony's post
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#4
Sorry, spaced out when writing that and corrected it now. The trans is a T56 Magnum Here's the gearing. 

[Image: image.png]

T56 Magnum

Currently have 275/60 15's on the back. That will change to a 17" or 18" wheel at some point, but about the same diameter.

I don't know what cam it has although I was told it has a mild cam and has a slight lope when idling which may or may not be from the cam.
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#5
(12-26-2018, 09:34 PM)73pony Wrote: I assume it is a 2.66 1st and a .63 OD?? I would go min 3.70 maybe all the way up to 4.11. TREMEC has an app you can download for free that allows you to calculate your speed and rpms with different rear gear ratios.

Grabbed the app, thanks! 

With 4.11's it's telling me I can go 41 in 1st and 203MPH in 6th @ 5000RPM! lol

Not really sure where I would really want to be for those lower gears.
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#6
(12-26-2018, 09:59 PM)TheDude Wrote:
(12-26-2018, 09:34 PM)73pony Wrote: I assume it is a 2.66 1st and a .63 OD?? I would go min 3.70 maybe all the way up to 4.11. TREMEC has an app you can download for free that allows you to calculate your speed and rpms with different rear gear ratios.

Grabbed the app, thanks! 

With 4.11's it's telling me I can go 41 in 1st and 203MPH in 6th @ 5000RPM! lol

Not really sure where I would really want to be for those lower gears.

Use this calculator. I would use either 3.91 or 4.11

https://www.blocklayer.com/rpm-geareng.aspx
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#7
(12-26-2018, 11:59 PM)barnett468 Wrote:
(12-26-2018, 09:59 PM)TheDude Wrote:
(12-26-2018, 09:34 PM)73pony Wrote: I assume it is a 2.66 1st and a .63 OD?? I would go min 3.70 maybe all the way up to 4.11. TREMEC has an app you can download for free that allows you to calculate your speed and rpms with different rear gear ratios.

Grabbed the app, thanks! 

With 4.11's it's telling me I can go 41 in 1st and 203MPH in 6th @ 5000RPM! lol

Not really sure where I would really want to be for those lower gears.

Use this calculator. I would use either 3.91 or 4.11

https://www.blocklayer.com/rpm-geareng.aspx

I went with the 4.11 gear and would not go back. Plenty of fun in the lower gears and can cruise down the highway at 70mph @ 22/2300 rpm. But the call is totally up to you, its your car.
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#8
(12-26-2018, 11:59 PM)barnett468 Wrote:
(12-26-2018, 09:59 PM)TheDude Wrote:
(12-26-2018, 09:34 PM)73pony Wrote: I assume it is a 2.66 1st and a .63 OD?? I would go min 3.70 maybe all the way up to 4.11. TREMEC has an app you can download for free that allows you to calculate your speed and rpms with different rear gear ratios.

Grabbed the app, thanks! 

With 4.11's it's telling me I can go 41 in 1st and 203MPH in 6th @ 5000RPM! lol

Not really sure where I would really want to be for those lower gears.

Use this calculator. I would use either 3.91 or 4.11

https://www.blocklayer.com/rpm-geareng.aspx

Ditto. My mild cammed 351C-4V liked the 3.91 gears very much....


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#9
Keep in mind that just because you might be able to go maybe 38 mph at 6,000 rpm etc, you will normally be shifting at around 60% of what the total rpm ability of the engine is.
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#10
Could you post pictures of what mods were done to trans tunnel, ect. to fit the T56? I always wanted one but was told trans would require cutting of floor for correct drive line angle.
I agree with others, a 3.91 rear gear or 4.11 would be perfect, 4.11 if going with a taller rear tire (28"" or more) in future.
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