Auto rear end ratio fix fix help

[SpiderJim]

I have just acquired the auto rear end that I will be installing on my 81 Spider Fi. From what I have read it seems the speedo will be off by around 8mph. Have any of you folks done this switch and have been able to correct the speedo difference. My current speedo is the 80 mph max type. I can get hold of just about any other type with higher readings. Does changing the speedo help or do I need to change to a different speedo drive in the tranny. One last question is does the auto tranny use a speedo drive that is compatible with the five speed so that I can pull it off and install in my tranny?

Thanks ahead of time.

[baltobernie]

The 3.56 axle was never installed in manual transmission cars, so you will need to "wing it" by mentally adjusting RPM to MPH after a drive with a GPS. You end up ignoring the speedometer, instead looking at the correct RPM to achieve a target speed. Or get an adaptor from a source like this: http://www.speedometershop.com/ratio.html#TOP. Make sure you are comfortable with your tire size before spending money for an adaptor

[TulsaSpider]

These speedos are not known for their accuracy anyways!

[So Cal Mark]

stick a gps in it, or get an adapter. Changing to a 140 speedo won't affect it

[Daniel]

you can adjust the speedo needle with long nose pliers and move the needle 8 MPH backwards did this on a
Fiat before . I Used the Pliers to hold the shaft of the speedo while i moved the needle/ pointer problem solved
of the most part .

[v6spider]

If you change the ratio of the rear end, the speedometer drive gear ratio needs to be changed as well. You can change the drive gear or buy a ratio adapter as someone suggested. But bending the needle of the speedometer will only cause the speedometer to be more inaccurate. When the drive ratio is out the level of inaccuracy changes exponentially as the speed increases.

Rob

[Daniel]

If you change the ratio of the rear end, the speedometer drive gear ratio needs to be changed as well. You can change the drive gear or buy a ratio adapter as someone suggested. But bending the needle of the speedometer will only cause the speedometer to be more inaccurate. When the drive ratio is out the level of inaccuracy changes exponentially as the speed increases.

I disagree with you Rob if i hadn't made a change like this and saw the results myself i wouldn't be sharing it
on a forum . If you can come up with a better fix i am sure it would be a great tool to solve this problem
but finding something over the counter is going to be impossible !

[v6spider]

I learn this from my local transmission shop and here's how you test it. On the highway when you are cruising take note of the odometer reading in tenths when you come across the first mile marker. Drive to the next mile marker and note the odometer tenths reading. It should be exactly the same as when you started at the first one. If it isn't then you need to change the drive gear that is in the transmission for the speedometer. The transmission shops have kits that have gears for different rear end ratios. I would suggest getting the speedo drive gear out of an automatic if that is the rear end that is being used. Talk to a transmission mechanic... he will verify what I am talking about. I had to deal with this when I put my rx7 rear end in my spider because of the change in gear ratios.

Here is a link to a page that somewhat explains what I am talking about:
http://www.transmissioncenter.net/speed ... ____va.htm

Here is a link to a drive gear calculator:
http://www.sccoa.com/faq/speedgr.html

Most of the info here is geared toward American cars. But the theory is the same. Any time you change rear end ratios or outside tire diameters( not rim size) or a combination of both, the speedometer drive gear will need to be changed. The easiest way to fix this would be with a drive ratio adapter. The first link I provided lists some of those. But I am sure if you google for it you will find more.

rob

[Daniel]

Since the Automatic tranny is GM ( fiat didn't have autos back then ) and the Manuel is Fiat i would
think swapping the gear drive might be something that needs to be modified to make this work but
i've never compaired them side by side .

[v6spider]

Daniel,
The easiest way I know to fix this is with a drive ratio adapter. Most of the vendors have online tool that will help you determine what adapter you need. Although compare them ...wont hurt...