Thinking of changing Rear ends ?

GOPAPA · Post by **GOPAPA** » Fri Jan 16, 2009 5:24 pm

As most of you know I have a 82 spider automatic with a rear end in it and I guess it's stock.. I was thinking that if it has a higher gear ratio I might want to put it in my 82 with a 5 speed and gain mpg plus slow down the engine rpm's at higher speeds.I seem to have all the power I need but the 5th gear seems a little to low of a gear..

I would also like to know if it will bolt right in or will the drive line have to be changed .. Ld

Ernie · Post by **Ernie** » Fri Jan 16, 2009 6:35 pm

GOPAPA,

Yes, it will work. I haven't done it, and don't own a Spider later than '78, but I've seen this modification written about a bunch.

One problem, though. Going from the 3.90 gears to 3.58 will make the speedometer wrong and if I recall correctly, the speedometer drive on the automatic turns opposite the one on the 5-speed, so you can't just swap them. Bummer.

Ernie

GOPAPA · Post by **GOPAPA** » Fri Jan 16, 2009 6:52 pm

Ernie wrote:GOPAPA,

Yes, it will work. I haven't done it, and don't own a Spider later than '78, but I've seen this modification written about a bunch.

One problem, though. Going from the 3.90 gears to 3.58 will make the speedometer wrong and if I recall correctly, the speedometer drive on the automatic turns opposite the one on the 5-speed, so you can't just swap them. Bummer.

Ernie

I wonder if I am saying it wrong from your answer.. I have a 82 parts car with a automatic
I have a 82 driver car with a 5 speed

I want to put the rearend only from the parts car into my driver car and stay with the 5 speed .. does that explain things better?

Ernie · Post by **Ernie** » Fri Jan 16, 2009 7:16 pm

I think I understood both times. Remember i said the problem you end up with is speedometer related.

Changing the differential ratio is like changing to tires of a greatly different diameter. I understand you want to go faster without the engine beating itself into never never.

When the rear end is the only thing changed, the speedometer will no longer be slightly inaccurate like it was, it will be a whole lot inaccurate.

Backing up a step -- if you put a transmission from a 4:10 rear-end car into a car that originally had a 4:30 rear-end you can fix the speedometer by swapping out the speedometer drives. Actually, you should swap the drive assembly and the gear on the transmission tail shaft. (big nut on the back end -- big trouble without a good impact wrench)

The problem with your scenario is that you are swapping an auto rear-end into a 5-speed car. There's no easy way to fix the speedo because the drive mechanism turns in reverse direction.

Someone feel free to correct me if I'm wrong. But this is my understanding of the differences.

Ernie

GOPAPA · Post by **GOPAPA** » Fri Jan 16, 2009 7:31 pm

Ernie wrote:I think I understood both times. Remember i said the problem you end up with is speedometer related.

Changing the differential ratio is like changing to tires of a greatly different diameter. I understand you want to go faster without the engine beating itself into never never.

When the rear end is the only thing changed, the speedometer will no longer be slightly inaccurate like it was, it will be a whole lot inaccurate.

Backing up a step -- if you put a transmission from a 4:10 rear-end car into a car that originally had a 4:30 rear-end you can fix the speedometer by swapping out the speedometer drives. Actually, you should swap the drive assembly and the gear on the transmission tail shaft. (big nut on the back end -- big trouble without a good impact wrench)

The problem with your scenario is that you are swapping an auto rear-end into a 5-speed car. There's no easy way to fix the speedo because the drive mechanism turns in reverse direction.

Someone feel free to correct me if I'm wrong. But this is my understanding of the differences.

Ernie

I need correcting also ,,because it seems to ,me that the speedo cable still would hook to the tranny and the tranny isnt going the opposite direction because of the rear end surely.. I'm confused or sompton ,, LOL as for the speedometer ,,,,ever since I been on this forum no one has addressed how you get the things to be purrely accrate ..so I came to the conclusion they aint never going to be accurare .. So I will do what I have already and thats have some one with a accurate speedo drive behind me and with our phones I find out just what speed I am going ,, Now I will get the feathers ruffled LOL
I

mdrburchette · Post by **mdrburchette** » Fri Jan 16, 2009 8:26 pm

Won't ruffle any feathers here, LD. If an incorrect speedo doesn't bother you that's okay. Mine's not right and all I did was put lower profile tires on the car....not that I look at the speedo much anyway!

Ernie · Post by **Ernie** » Fri Jan 16, 2009 9:13 pm

No feathers ruffled here either. Just trying to help.

Again, I believe changing the differentials or the entire rear axle will work just fine in your case because they are late differential to late differential.

You're right -- the transmission isn't changing and the speedometer drive isn't changing. What will change is the speedometer reading. If you can live with that you should go for it.

What I meant when I spoke (typed) of the speedometer drive is that you don't have the opportunity to fix the speedometer error by simply changing the drive mechanism.

There are shops that will sell you some sort of geared converter that splices into your speedometer cable to fix the error. I get the idea it's about a $100 fix.

Ernie

GOPAPA · Post by **GOPAPA** » Fri Jan 16, 2009 9:17 pm

mdrburchette wrote:Won't ruffle any feathers here, LD. If an incorrect speedo doesn't bother you that's okay. Mine's not right and all I did was put lower profile tires on the car....not that I look at the speedo much anyway!

Oh ! was about to ruffle feather ?? LOL ,, but you brought up something that i would like to do ,,,and thats put some wider tires on my car ..to me it would take away that model T look and make it look like a sports car .. and as far as the speedo thing , If I can figure out how much it is off then I will deduct or add and keep on speeding down the road..

GOPAPA · Post by **GOPAPA** » Fri Jan 16, 2009 9:21 pm

Ernie wrote:No feathers ruffled here either. Just trying to help.

Again, I believe changing the differentials or the entire rear axle will work just fine in your case because they are late differential to late differential.

You're right -- the transmission isn't changing and the speedometer drive isn't changing. What will change is the speedometer reading. If you can live with that you should go for it.

What I meant when I spoke (typed) of the speedometer drive is that you don't have the opportunity to fix the speedometer error by simply changing the drive mechanism.

There are shops that will sell you some sort of geared converter that splices into your speedometer cable to fix the error. I get the idea it's about a $100 fix.

Ernie

I hear and see what you are saying or typing ,, thanks for the reply and my thoughts are Yes I can live with the speedo incorrectness.. As I was telling Denise ,,when I change tires to a lower wider profile it will make another change in the accuracy anyway .. have to wait and see.. Ernie we had a good chat friend LD

Ernie · Post by **Ernie** » Fri Jan 16, 2009 10:13 pm

Agreed, LD. Sleep well, my friend.

Ernie

lanciahf · Post by **lanciahf** » Fri Jan 16, 2009 10:44 pm

My 5sp Spider has an auto rear. The drive is pretty relaxed and it still accelerates well. With Fuel Injection it hardly uses any fuel. I would love to get a 4.10 rear but they are sort of non-existent. My car started as an Auto originally.

4babycar · Post by **4babycar** » Fri Jan 16, 2009 11:28 pm

You can also by-pass some of this by putting in one of the cheap GPS navigators, they display your current speed and direction for the same $100. Plus they can tell you where the nearest Starbucks is located too!

racydave · Post by **racydave** » Sat Jan 17, 2009 10:55 am

The automatic rear-ends are probably good for us flatlanders. But maybee not for hilly driving.

So Cal Mark · Post by **So Cal Mark** » Sat Jan 17, 2009 11:33 am

the downsides of the auto rearend are noticeably reduced acceleration at low speeds, and (depending on your cruising speed) the possibilty of highway speeds at less than peak torque. If you drive normally at 60 mph, I'd leave it as is. If you just want a freeway flyer for cruising at 80mph, then there isn't a downside at all

GOPAPA · Post by **GOPAPA** » Mon Jan 19, 2009 9:14 pm

Good way to put it Mark ,,I do not care about high speeds ,just thought of RPM's and how the motor is wound up at 70 mph..
And like some one said I live up a 7% grade hill that is a mile long too..anyway the car is going to have to sit awhile anyway as I have to take care of some medical issues ,, so you and everyone keep up the good forum cheer with help for us who need it and laughs for all who like it .. LD

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