Recommended Rear End Ratio - 6 Speed

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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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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?

 
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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.

 
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Sorry, spaced out when writing that and corrected it now. The trans is a T56 Magnum Here's the gearing. 



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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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.

 

barnett468

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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

 

Tnfastbk

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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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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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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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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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The low end performance of the engine is a big factor in happy overdrive gearing. My 2000 Viper has a .5 sixth gear and with a 3.07 ring and pinion. Sixth can be used at 60 mph quite easily. I have the same trans in a '72 Vette and a 3.55 final drive. When it had a 427 in it with a 246 @ 50 cam, 70 MPH was about the slowest it was happy in sixth. When it had a mild 350 with a 196 @ 50 cam it was easy to use sixth down to about 63 MPH. I have an 11.5 :1 compression 406 in a Camaro that required a slower advance curve when I installed a TKO600 in it. It had been fine with the 1:1 four speed but even with the slower curve, 2000 rpm is about as low as I take it in fifth.

After doing six overdrive conversions in muscle cars I've found that carb and ignition tuning can get very picky as you try to use numerically lower overall ratios especially with any kind of performance cam and compression. There are a lot of hills where I live and that makes tuning even more important in trying to avoid constant downshifting. If it's flat where you live it will be easier to use low rpm/OD combinations. Heat comes into play too. Adding OD to older cars can put them into a situation (high load/low rpm) that is a asking for detonation.

 
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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.
OK cool, the 4.11's are sounding like the way to go. Now I just need to figure out how to get that done. 

There were no mods needed to the tunnel other than opening up the hole for the shifter a bit, but even that was pretty minor. I've also read that there were tunnel mods needed but there were no clearance issues while getting the right angle. It's tight, but it fits fine on my car.

 

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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.
Dude,  you might want to a driveshaft speed calculation once you figure out gearing  and tire size. pretty sure you are going to need a custom driveshaft anyways (for the length ) but it is going to have to be a larger diameter too and probably of a different material.                                      boilermaster.

 
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With lower gears and a 0.50:1 ratio overdrive your driveshaft will be spinning much faster at a given speed. It will be spinning at 2 times the engine revolutions when in 6th gear.

Check with your driveshaft builder to make sure it can handle your new transmission and rear end and is balanced for the higher RPMs.

 
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.50 OD!

That is kind of nuts.

You can figure out the critical speed for the driveshaft you have by looking at the chart posted on the link below.

With your rear end gear ratio and tire size you should figure out what critical speed would be in MPH. My guess is that once you figure that out the number will be something ludicrous like 140mph or above. At that point you have to worry about tires and life insurance too, so you will probably be fine with the driveshaft you have.

Whatever critical driveshaft speed works out to be you might want to keep that in mind if you ever take it to a chassis dyno where you could get it whipped up to ludicrous speed. Just keep it under critical speed....

http://pstds.com/critical-speed-chart/

 
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I have a low rust/lots of dents '71 sportsroof.
.50 OD!

Whatever critical driveshaft speed works out to be you might want to keep that in mind if you ever take it to a chassis dyno where you could get it whipped up to ludicrous speed.  Just keep it under critical speed....

http://pstds.com/critical-speed-chart/
I've always been in 1:1 for chassis dyno runs.

 

boilermaster

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Working on a buddy's high end 69 cutlass right now with a tremec, ford 9'' and 4 link that has a vibration @ 90 mph. absolutely everything is new.                                                                            after checking the runout on his custom driveshaft, I came up with .003'' runout.                                No problem, I will just adiust the pinion angle with the 4 link.                                                           While trying to get the 2 angles to be parallel, I realized his coil springs were about to come off their seats and the links were about bottomed out.                                                                              The end result, either cut off all of the suspension mounts off of the housing and re weld or get another custom housing. the existing housing had already been powder coated.                             Why did this happen, you might ask ?                                                                                         The dude that built the rear axle did not take into account that the Ford 9'' pinion sits about .750'' lower than it's GM counterpart and he did not ask what transmission was being used.                     When I last talked to my buddy, he stated that the diff dude is supplying another housing with all attachments unweleded, we will mock up, tack weld and send it back to him for final welding and powdecoating,   HOMEWORK fellas do your homework first.                                                          Boilermaster

 
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