3.58 Gear Ratio, pre '79

[ljayr]

I have a 1979 Spider 2000. I assumed that it had a 3.90 rear end in it, well I assumed wrong. The previous owner changed the rear end with a pre '79, making it a 4.10 gear ratio. This gear ratio sucks at any speed over 65MPH.

With this year rear end, the only gear ratio option is the 4.10.

Looking at the later differentials I found that there were more options. In the standard transmission, there was a 3.90 rear differential and in the automatic a 3.58 differential.

I am going to opt for the 3.58 gear ratio.

The 3.58, nor the 3.90 is an option in the pre '79, but I am going to make it a viable option fro now on.

If I am not the 1st one to do this please let me know, before I get to far involved in this project and determine it not-feasible.

If I am the 1st, then shame on all of you for not trying this earlier.

Either way, stay tuned for pictures and complete instructions.

Jason

[124JOE]

i feel the shame
well i dont have parts to try it
TUNED IN

[azruss]

I went the other way putting a 71 rear end in a 80 FI. If your goal is to have a hiway cruiser without any passing guts, then you are going in the right direction. I drove a stock 70 for over a decade and got tired of having to gear up to get any power. When i dropped a 2L carb in the same package, i was very pleased with the performance and flexibility of acceleration at any RPMs. I didnt mind the hi hiway revs because of how well the motor handles it. Before you go to an auto rear end, you need to be okay with trading the powerband location. Not convinced you would save that much in gas either. I had a volvo with an overdrive and on hilly freeways, i got better performance and economy in 4th gear where the motor was in its sweet spot and didnt have to work so hard.

[spiderrey]

A 358 rear only works well if you have a built 2 liter.I cant imagine having that rear end set up on a 70 with a 1438, that would be gutless. I had the 358 on my 70 but with a built 2 liter. Im swapping back to a 410 or a 390 if I can find one first. i have 2 410,s but would like to try a 390 first.

[So Cal Mark]

you've got your years/ratios a bit mixed up. 79 and newer cars had 3.9 with a manual and 3.58 with an auto. 1800 cars had 4.30 and the early cars had 4.10. I usually advise against the 3.9 swap, but then I don't obsess with 4k on the freeway either. With my mods, the torque curve is wide and flat and the motor is really happy buzzing along at that speed

[DocGraphics]

My 78 has the newer style rearend where the rear cover comes off, not the pumpkin dropping out the front, does that mean I would have the 3.90 gearing???

[So Cal Mark]

if you have the removeable rear cover you have either 3.9 or 3.58. Like everything else, late 78s started getting some of the 79 stuff. Or someone may have swapped diffs

[Exit98]

Jayson,

I went from a 4.30 to a 3.90 in my 76 Spider and I couldn't be happier. You need the driveshaft and panhard rod from the donor car and some other stuff but it is an easy swap. PM me if you want details.

From 4200 rpm to about 3750 at 70 mph. But the real improvement is around town.

[ljayr]

Hey guys, thanks for the feedback. So here is what I have so far.
Lets start by taking a look at the pinions. The one on the left is post '78 and the one on the right is pre '77.
I started by measuring all of the heights and diameters. It turns out they are equal. The only difference in the two pinions are the top thread size and the splines.


So to deal with the different splines I will need a different front flange. Here are the two different styles of flanges.

It turns out that the seal diameters and heights are equal. The only difference is the overall length of the flanges. Hopefully I will have enough clearance in the drive line to deal with this height difference. The one on the left is the pre '77 while the one on the right is post '78.

The rings are identical except for the number of teeth.


Unfortunately you cant just interchange the carriers. There is quite a bite of difference between the two.

The one on the left is the post '78 and the one on the right is pre '77. All I'll have to due is swap the ring gears.

Well it looks like I have a good deal of part cleaning to do before re-assembly.

I'll keep you guys posted with my progress.

Jason

[124JOE]

thanks for the pics there great

[vandor]

if you have the removeable rear cover you have either 3.9 or 3.58. Like everything else, late 78s started getting some of the 79 stuff. Or someone may have swapped diffs

Mark, the '78s with the newer style diff still had the 4.3:1 ratio, it only changed to 3.9 with the Spider 2000 in 1979.
I've had several '78s with that rear end, and I wish they were 3.9:1!

[vandor]

With this year rear end, the only gear ratio option is the 4.10.

Actually most of those were 4.3:1, but there are 3.9:1 gears available for the early rear end.
The early rears are 20 lbs lighter and seem to fail less often, so many people prefer to stay with them.

[DocGraphics]

Csaba, do you know the tooth count on the ring gear for the 4.3 & the 3.9, I'll have the rear cover off my 78 (10/77 w/ newer style rearend) cleaning up my rearend this spring & would like to verify what I have. & if it is the 4.3 can I just change the ring & pinion to get the 3.9 ratio & then change the speedo gear to match so it all works right.

[ljayr]

Thanks for catching that mistake of mine. The 4.1 should be a 4.3. My understanding of the gear availabilities is this:
1970-77
10/43 (pinion/ring)
1978-85
10/39 yrs 1/78 to 8/79
12/43 yrs 9/79 on

Utilizing any of the online gear calculators, I should see between 800-1000 RPM drop at cruising speed. I think the 2.0L can handle it. Either way, I will find out.

[maytag]

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