Speedometer ratio question

[bergie33]

I have a couple different speedometers, one is 1.297w and one is 1.393w.  I will have a 3.91 diff when I'm done, so I want the 1.393 Speedo. I don't know the condition of the speedos, and to be honest, if the rear plate of these speedometers was ever replaced then who knows what ratio they really are.

 

So, I have a Speedo cable hooked up to a drill. The 1.393 Speedo reads 75 at max speed and the 1.297 reads 70.  I'm trying to find out if that seems to be a reasonable difference between the two different speedo ratios, thereby confirming one is a 1.297 and the other is the 1.393?

 

I know there was a speedo face overlay replacement that BMW did with the 76's. Was it some or all of them? Both of my speedos have the same face, so not sure if that plays into it at all. 

[jimk]

1 hour ago, bergie33 said:

So, I have a Speedo cable hooked up to a drill. The 1.393 Speedo reads 75 at max speed and the 1.297 reads 70.  I'm trying to find out if that seems to be a reasonable difference between the two different speedo ratios, thereby confirming one is a 1.297 and the other is the 1.393?

The numbers represent the number of driveshaft turns per mile.  If you calculate the wheel turns per mile, then it will tell the differential ratio that each was intended for use with.

 

Keep in mind, what you perceive as a decimal point is a comma in Europe.

Edited February 25 by jimk

[Son of Marty]

Not a whole lot of 76 speedo's got the overlay before BMW made the switch to the 1.39 ratio to match the 3.90 rear end. The 1.39 speedo with the 3.91  rear end will fall well with in the error of the system. The chart was ripped off from a old post by Les.

List (below) taken from a post earlier this year.

3.45:1 diff -- W=1.24 speedo

3.64:1 diff -- W=1.297 speedo (W=1.30 is close enough)

3.90:1 diff -- W=1.39 speedo ('76 49-State 2002)

3.91:1 diff -- W=1.393 speedo (E21 320is LSD)

4.10:1 diff -- W=1.4 speedo (1.461 calculated)

4.11:1 diff -- W=1.4 speedo (1.465 calculated)

4.38:1 diff -- W=1.559 speedo (calculated)

[bergie33]

3 hours ago, jimk said:

The numbers represent the number of driveshaft turns per mile.  If you calculate the wheel turns per mile, then it will tell the differential ratio that each was intended for use with.

 

Keep in mind, what you perceive as a decimal point is a comma in Europe.

 

Ok, gotcha.  I never knew exactly what the 1,393 and 1,297 referred to. Thanks. And by driveshaft, I assume you mean the Speedo driveshaft and not the transmission driveshaft, correct? 

[jimk]

55 minutes ago, bergie33 said:

And by driveshaft, I assume you mean the Speedo driveshaft and not the transmission driveshaft, correct? 

You are correct.  The speedo drive gear has a ratio of 2.5 main driveshaft turns to one speedo cable turn (2.5 :1).

 

The stamped numbers are used to calibrate the speedo at 60 mph if the spindle is turned at that rpm.

 

The stamped number x 2.5 also represents the engine rpm at 60 mph in 4th gear with oe tire rolling diameter. 

 

[Conserv]

On 2/24/2024 at 9:54 PM, Son of Marty said:

Not a whole lot of 76 speedo's got the overlay before BMW made the switch to the 1.39 ratio to match the 3.90 rear end. The 1.39 speedo with the 3.91  rear end will fall well with in the error of the system. The chart was ripped off from a old post by Les.

List (below) taken from a post earlier this year.

3.45:1 diff -- W=1.24 speedo

3.64:1 diff -- W=1.297 speedo (W=1.30 is close enough)

3.90:1 diff -- W=1.39 speedo ('76 49-State 2002)

3.91:1 diff -- W=1.393 speedo (E21 320is LSD)

4.10:1 diff -- W=1.4 speedo (1.461 calculated)

4.11:1 diff -- W=1.4 speedo (1.465 calculated)

4.38:1 diff -- W=1.559 speedo (calculated)

As @Son of Marty says, very few ‘76 models ever received the speedometer overlay. It was a dealer installed “fix” for speedometers that were marked 1.39 by the factory but actually contained 1.297 gears. My ‘76 (VIN 2742541, April 19, 1976) has a speedometer marked 1.39, but with 1.297 gears. After reading about the problem in two consecutive issues of the Roundel, I asked my dealer if the problem might apply to my car. They replied, “Of course not, that problem was fixed long before your car was built!” 🙄

 

The truth was that no one knew how many mis-marked speedometers were produced. And, before GPS, there was no easy way to verify your speedometer’s accuracy short of getting a ticket. After GPS, I discovered my speedometer was off by nearly 10%, crushing my belief that my bone-stock ‘02 had cruised between 116 and 120 mph on the New York Thruway from Rochester to Syracuse… 😟

 

Regards,

 

Steve