engine and drivetrain Which Diff Ratio?

[crumble]

Hi 2002faqers...

 

First post for me.

I have a '74 2002 which I inherited from my Dad.

I know he replaced the diff at some stage.

This caused the speedometer to read faster than you are actually going.

At 60 km/h the speedo reads closer to 80 km/h

Where does the speedo cable 'read' from?

How can I make the speedo read correctly?

[Conserv]

The speedometer cable is driven off the transmission. If you change the rear end ratio, you can change the speedometer head in your instrument cluster -- each head has its gearing stamped into the reverse side -- or you can have a speedometer shop modify your existing head. I would presume they would change the gearing, but they may have other means to adjust the speed readout as well.

There are lots of threads on the forum addressing this issue. You might try searching on something like "speedometer gearing" or "speedometer ratio."

The size of your speedometer error -- 30% at 60 km/h -- however, is exceptionally large, and difficult to account for given "the usual" differential swaps, e.g., replacing a 3.64 with a 3.91, or even replacing a 3.45 with a 4.10. Are your tires a stock size, or approximately a stock size in diameter, e.g., 23.2"?

Steve

[saaron]

Others may chime in...the only way I know to correct that is to send it to a place like North Hollywood Speedo to have it recalibrated.  Or you could try to find a speedo that matches the diff ratio.  

 

If you search, you can find info in posts that shows which speedo goes with each diff ratio - there are numbers on the back of the speedo that you use for this exercise.  

 

It would be helpful if you knew the diff ratio.  

 

Scott

[saaron]

Others may chime in...the only way I know to correct that is to send it to a place like North Hollywood Speedo to have it recalibrated.  Or you could try to find a speedo that matches the diff ratio.  

 

If you search, you can find info in posts that shows which speedo goes with each diff ratio - there are numbers on the back of the speedo that you use for this exercise.  

 

It would be helpful if you knew the diff ratio.  

 

Scott

[ray_]

I assume you are not in USA. But North Hollywood Speedometer can calibrate your speedometer, and what they ask is that you count the speedometer cable revolutions over a certain (small) distance (while rolling the car).\

 

It has been my experience that older speedos tend to read high....

 

Good luck,

[Hans]

It may be cheaper to find another speedo.

[ray_]

It may be cheaper to find another speedo.

But I'd give the probability of accuracy about 5%.

 

:-)

 

Cheers,

[gaius]

or even replacing a 3.45 with a 4.10

 

is there such a thing as a 3.45 differential, i thought 3.64 was the lowest they went.

[TobyB]

There was a 3.45 in the turbo, I heard.

 

I've never seen one up close and in person.

 

The 318i E36 got one in a small case, too, as the 

trans is a non- od 5 speed.  I have one of those in a 318i.

 

There are other low numbers floating around for the 168mm case,

but they are suspiciously the same as the numbers for the 188mm case,

which got 3.23s, 2.93's 2.73's etc.  Problem is, after the E21,

you could put a mid case into an E30 or E36 easily, so the numbers

seem to have leaked over, with people claiming they were small case gears.

 

I think.

 

t

[Conserv]

is there such a thing as a 3.45 differential, i thought 3.64 was the lowest they went.

Some early tii's also used 3.45 diffs. It's not clear if these were only delivered on Euro cars but there are '71-'72-ish examples on these shores. Sehr gut auf der Autobahn!

Steve

Edited November 16, 2014 by Conserv

[gaius]

hmmm, would like to have one of those.

Edited November 16, 2014 by gaius

[gaius]

so the 318i E36 case, is the same/interchangeable with a 02 ????

[ray_]

is there such a thing as a 3.45 differential, i thought 3.64 was the lowest they went.

BenT has one in an original Euro tii.

 

Cheers,

[gaius]

BenT has one in an original Euro tii.

 

 

but he would probably notice if i tried to swap them out in the middle of the night. 

[M3M3]

so the 318i E36 case, is the same/interchangeable with a 02 ????

No.