3.58 Gear Ratio, pre '79

[aj81spider]

I can't comment on your pictures. However I have a 4.3-1 and I'm not a fan. I'm shifting from first to second about halfway through the intersection and I'm over 4000 RPM at 70 mph. If I could convert to the 3.58 I would in a second.

[maytag]

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[spiderrey]

76 didnt have an automatic. I think if you have a 4.3 rear end and you rotate the tire once, the drive shaft will rotate 4.3 times. Or it might be the other way around.

[maytag]

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[So Cal Mark]

I love the 4.30 ratio, I wouldn't have anything else

[maytag]

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[124JOE]

yes ive had the 4.3 up to 100mph bearly
it was screeming 5,500 on the tac
and wouldnt go any faster

these are made for good take off not top end speed

[FiatMac]

so, I'm not a math guy. My brain shuts down. So somebody help me here:

chocked one wheel. turned the input at the diff 10 times. here's what I get.

the one currently IN the car (that my gut says was for a '76 Automatic) gets 4.5 turns of the wheel for every 10 turns of the driveshaft.

the one I have that came from the '78 gets 5.25 turns at the wheel for every 10 turns at the driveshaft.

What does that make the ratios? whatever it is, the one currently installed is the lower-geared, and I've been happy with it. So I'll probably refurb it and keep it. I'll need to find a panhard bar, as the current one is 'dun'. (I might toy with making an adjustable-length version out of it)

Anyone know the procedure for replacing bearings & seals at the axle for the non-removable-cover type? I mean, what holds-in the axles? Do I have to pull the pumpkin?

Here’s the math.

The gears in a differential are arranged such that the average speed of the two wheels multiplied by the gear ratio is a constant. In equation form this would be:

Differential Input Turns = Gear Ratio(W1 + W2)/2

Where Wl and Wr are the revolutions of the Left and Right wheel respectively.

So if the rear end ratio is 3.58:1 and we are traveling in a straight line with both wheels turning at the same speed W, then for each 10 turns of the input we can solve the equation by substitution:

10 = 3.58(W + W)/2 = 3.58(W)

Or

W = 2.79 turns

If we are trying to determine the rear end ratio by chocking(holding stationary) one wheel while counting the turns made by the free wheel with 10 turns of the input shaft, then the equation becomes,

10 = 3.58(W + 0)/2 = 3.58W/2

Or,

W = 5.59 turns

Below is a list of this calculation for each of the available ratios.

Rear Ratio /Free Wheel Turns per 10 turn input
4.3 / 4.65
4.1 / 4.88
3.9 / 5.13
3.58 / 5.59


Maytag,
Not sure how accurate you were with your count, but the 4.5 count for the ‘76 is very close to the count above for a 4.30 rear ratio. The 5.25 that you got for the ’78 is closest to the count above for the 3.90 rear ratio. You can get better differentiation between the numbers if you use a 20 turn input count.

[maytag]

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[So Cal Mark]

if your motor tops out at 5500 rpm you have more problems than rear gear ratio

[DocGraphics]

Fiatmac

Thank you, I will be using this to figure mine out too!

[ljayr]

The '76 is a 4.3.
The '78 is a 3.58 from an auto or a 3.90 from a 5 speed. To verify, remove the rear cover and check the ring gear. The gear ratio will be stamped on the side of the ring gear.
As far as the panhard bar mounting goes, I dont know. You would have to check with someone that has been around these cars for a much longer time than I have.
100 MPH with a 4.3, doubtful. 3.90 or 3.58 possible.

Jason

[boogiedude]

yes ive had the 4.3 up to 100mph bearly
it was screeming 5,500 on the tac
and wouldnt go any faster

these are made for good take off not top end speed

I've got a 4.3 in my car and I've taken it up over 6grand in fifth. I didn't hit 100. I don't think your speedo is accurate, I know mine is up into the 70's(I've checked it on speed signs )

[spiderrey]

With stock gears and engine my old car topped out at 108 mph.

[vandor]

In my 76, I found this:

I assume you know that the panhard rod is seriously bent and needs replacement?