WTB late model speedo for 3.91 diff

[williamggruff]

I recently replaced my 3.64 diff with a 3.91 from an e21. I want to replace the speedometer/odometer gauge so that it accurately reflects driving speed. Specifically, I wish to obtain a working speedometer/odometer gauge with either a W=3.9 ratio or W=3.93 ratio, which will be stamped into the back of the gauge. 

 

PayPal ready. Also willing to trade. I have a LOT of parts.

 

Grice

[Conserv]

Grice,

 

Just a note of caution. The '02 most likely to have a speedometer head for a 3.90 diff (obviously very close to the e21 3.91) is a 1976 49-state version, which is what my '76 is (photos below of my instrument cluster and speedometer). Their speedometer heads are marked W=1,393, as mine is.

 

But...just be aware that a large batch of these speedometers -- including mine, which is dated March 1976 ("3 76") -- actually received the gearing appropriate to the more common 3.64 diff. My speedometer error is consequently between 9% and 10% with stock 165/80/13 tires and the original 3.90 diff. I'm not aware that any one has isolated these incorrectly-geared speedometers by VIN or date. The issue appeared at least twice in the Roundel (photos below of the notes posted in the Roundel).

 

So there is some uncertainty when you purchase a speedometer marked W=1,393 whether it will be, in fact, geared as a 1,393 unit.... ?

 

Regards,

 

Steve

 

 

 

Edited May 25, 2018 by Conserv

[williamggruff]

Thank you, Steve!

 

I believe my current speedo is a w=3.93 with the 3.64 gearing. My car is an April 76. The speedo was pretty accurate with the 3.64 diff :-(

[ray_]

5 hours ago, williamggruff said:

I recently replaced my 3.64 diff with a 3.91 from an e21. I want to replace the speedometer/odometer gauge so that it accurately reflects driving speed. Specifically, I wish to obtain a working speedometer/odometer gauge with either a W=3.9 ratio or W=3.93 ratio, which will be stamped into the back of the gauge. 

 

PayPal ready. Also willing to trade. I have a LOT of parts.

 

Grice

 

Why don't you just get it re-done at NH Speedo instead of taking a chance?

 

They'll have you manually measure speedo cable rotations over a mile and match your speedo to that exactly.*

 

That will of course also address any tire circumference issues you may have caused ;-) and you'll get a refurbed assembly.

 

Cheers,

 

*okay it's less than a mile

Edited April 22, 2018 by ray_

[williamggruff]

I don’t think I can count the speedo cable revolutions while driving. Seems like a recipe for disaster. Or a watery, bitter bouillabaisse. 

[ray_]

3 minutes ago, williamggruff said:

I don’t think I can count the speedo cable revolutions while driving. Seems like a recipe for disaster. Or a watery, bitter bouillabaisse. 

 

Marshall's driveway is long enough.

 

[williamggruff]

LOL. My driveway is about 700 feet long. I suppose it could drive a fraction of a mile on ththe driveway and multiply my results by 4/3PiR^3€£•Newton/sec/sec. 

[ray_]

4 minutes ago, williamggruff said:

LOL. My driveway is about 700 feet long. I suppose it could drive a fraction of a mile on ththe driveway and multiply my results by 4/3PiR^3€£•Newton/sec/sec. 

 

Yes. I was teasing about the mile.

I measured mine in a hotel parking lot. It was 60 or 70 feet recommended by NH speedo. I forget the exact distance. Or maybe 165.

 

 

Also your equation is missing furlongs, smoots and fortnights. 

Edited April 22, 2018 by ray_

[Conserv]

8 hours ago, ray_ said:

 

Why don't you just get it re-done at NH Speedo instead of taking a chance?

 

They'll have you manually measure speedo cable rotations over a mile and match your speedo to that exactly.*

 

That will of course also address any tire circumference issues you may have caused ;-) and you'll get a refurbed assembly.

 

Cheers,

 

*okay it's less than a mile

 

Yeah, what he said... ?

 

Best regards,

 

Steve

 

[jimk]

Your turns per mile will be:

Wheel rotations per mile x dif ratio / 2.5

 

2.5 is the reduction ratio of the transmission speedometer gear.  Turn per mile "W" is always > 1000.  Look close and there will be a faint 1 ahead of the 393.  Of course take note the the european marking will be a :," where we Yanks use a "."

 

Edited April 23, 2018 by jimk

[rstclark]

Had my speedo redone by NH Speedo and calibrated for rear end and tires. As I recall they asked me to flag the speedo cable and count the revolutions for 50 feet   Easy to do.  Just tape off 50 foot distance on the street and try it a few times to get a good average of the counts.

 

[Conserv]

14 hours ago, rstclark said:

Had my speedo redone by NH Speedo and calibrated for rear end and tires. As I recall they asked me to flag the speedo cable and count the revolutions for 50 feet   Easy to do.  Just tape off 50 foot distance on the street and try it a few times to get a good average of the counts.

 

 

And...how do the speedometer’s readings now compare to GPS readings?

 

Regards,

 

Steve

 

[rstclark]

Perfect with the radar speed signs

[David Layton]

My 73tii has a late instrument cluster and I have recently acquired a model correct cluster.  The speedo is marked 'W=1,24".  The car has a 320i OD 5 speed and the LSD.  So the late cluster may have been added for this reason.  

 

We have a local speedometer shop in the St. Louis area that can work on this VDO.  I want him to go over it and calibrate it for my running gear.

 

Here is my calculation:

 

Tires - Pirelli CN36 185/70 r13

 

Tire Diameter per Pirelli- 23.18"

 

I am coming up with W=1.361.  Speedo shows W=1.24

 

See Attached Math Homework

 

 

73 BMW 2002tii - Speedometer Calibration Calculation.xlsx