Speedometer calibration/Rear axle ratio/point of origin

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ES 1800

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'73 CS

I do not know the history of my car. It had a metric (kph) speedometer in it, but based upon the discoloration of the dial vs. other instruments I had suspected that it was not the original. The odometer stopped working so I decided to replace it with the US (mph) speedometer in my stock. Three days later ... the speedometer reads way high.

I read the thread for 2016 about changing the other way, including rear axle ratios. Good information.

Are you all saying that Hollywood can change a speedometer's calibration?

Do 3.0 CSs not have a speedometer gear in the transmission?

How does one know if their car was imported into the US orginally or if it is a "UK" model (as I have seen on the wiring diagrams of the repair manuals I have.

I am not looking forward to repeating the process, although it's always easier the second time.
 
Jim,

What is your VIN, that will tell a lot. Any Speedo shop can recalibrate a coupe speedo, mine reads 10% slow as I have a 3.45 diff vs. standard 3.64 (5% itself). Your trans has a mechanical drive and cable.
 
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HB Chris said:
Jim,

What is your VIN, that will tell a lot. Any Speedo shop can recalibrate a coupe speedo, mine reads 10% slow as I have a 3.45 diff vs. standard 3.64 (5% itself). Your trans has a mechanical drive and cable.
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VIN is 2240757.

Truth of the matter is that I have only one BMW so I have pretty limited knowledge of where to find things. I restore vintage Volvos. I own, maintain and drive a '62 P1800 race car. I know them inside and out. BMWs, not so much, so I appreciate the help suggestions about where to find info.
 
Hoffman delivered US manual transmission coupe (224xxxx). A speedo shop will change the gear inside the speedo, I use Waze to get true speed and compare to speedo. Mine is off 10%, tell the shop. Some want you to count how many revolutions of rear wheel over a distance of 50', same thing.
 
ES 1800 said:
'73 CS

I do not know the history of my car. It had a metric (kph) speedometer in it, but based upon the discoloration of the dial vs. other instruments I had suspected that it was not the original. The odometer stopped working so I decided to replace it with the US (mph) speedometer in my stock. Three days later ... the speedometer reads way high.

I read the thread for 2016 about changing the other way, including rear axle ratios. Good information.

Are you all saying that Hollywood can change a speedometer's calibration?

Do 3.0 CSs not have a speedometer gear in the transmission?

How does one know if their car was imported into the US orginally or if it is a "UK" model (as I have seen on the wiring diagrams of the repair manuals I have.

I am not looking forward to repeating the process, although it's always easier the second time.
Click to expand...
Speedometers from 2.5CS, 2800CS / 3.0CS, 3.0CSI/CSL - have different gears ratios inside speedometer.
You can find that ratio information on back wall/side of speedometer.
 
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Wladek said:
Speedometers from 2.5CS, 2800CS / 3.0CS, 3.0CSI/CSL - have different gears ratios inside speedometer.
You can find that ratio information on back wall/side of speedometer.
Click to expand...

It all depends on the diff ratio, 3.25, 3.45 or 3.64 but even when new I don't think they were all that accurate and many times diffs have been changed.
 
ES 1800 said:
I just posted at http://www.e9coupe.com/forum/threads/changing-my-speedometer-from-mph-to-km-whats-wrong.23254/page-2. Looks like I have a 3.64 closed diff. Speedometer is marked W=1.207.
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At first check diff ratios - they are marked on the bottom right side of diff under axle mount (this place might be very dirt and markings might be not visible).
I think you will have factory 3.0CS 3.45 diff (38:11). That's probably why you have weird speedometer readings.

p.s. closed diff - it is LSD? Or fully locked (welded open)?

HB Chris said:
It all depends on the diff ratio, 3.25, 3.45 or 3.64 but even when new I don't think they were all that accurate and many times diffs have been changed.
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That's right Sir! I did not mention it before because i thought it was obvious.
 
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Wladek said:
At first check diff ratios - they are marked on the bottom right side of diff under axle mount (this place might be very dirt and markings might be not visible).
I think you will have factory 3.0CS 3.45 diff (38:11). That's probably why you have weird speedometer readings.

p.s. closed diff - it is LSD? Or fully locked (welded open)?


That' right Sir! I did not mention it before because i thought it was obvious.
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i meant open diff. Not LS. It's 3.64.
 
Is your transmission a 5 speed with 5th and overdrive? IF it is, then the transmission output is set for a 3.07 diff in the car it was removed from. About 19% difference in speed of the vehicle between a 3.07 and a 3.64. My car has the OD and had a 3.64 diff which I changed to a 3.07. Speedo was ~20% fast, it is now spot on when compared to my GPS speed. My car has a upgraded engine that is ~275 hp, and it works well with the 3.07 as a cruiser - not so good as a drag racer. Stock engine output with a 3.07 would make a pretty slow accelerating car.
 
1.2 equates to a 3.64, coupes didn't come with a 3.07. You are correct though that the difference between 3.07 and 3.64 is about 20%. Makes no difference if it's a 4 or 5 speed.
 
Gary Knox said:
Is your transmission a 5 speed with 5th and overdrive? IF it is, then the transmission output is set for a 3.07 diff in the car it was removed from. About 19% difference in speed of the vehicle between a 3.07 and a 3.64. My car has the OD and had a 3.64 diff which I changed to a 3.07. Speedo was ~20% fast, it is now spot on when compared to my GPS speed. My car has a upgraded engine that is ~275 hp, and it works well with the 3.07 as a cruiser - not so good as a drag racer. Stock engine output with a 3.07 would make a pretty slow accelerating car.
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Mine is a 4 speed awaiting a 5 speed conversion.. One of those "down the list projects."
 
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