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Discussion Starter #1
Does anyone know (or is there published literature) on the relationship of the stock, intended, actual rpm of the spedo cable to the speed displayed on the face of the gauge? I have an abnormale situation which I though was solved by other means, however, that is not working.

Ideally I need to figure out how to have output from a T-5 spedo cable drive feed a stock spedo. Hence the inquiry about the desired rpm of the spinning cable. On the T-5 there are lots of drive speed gear alternatives. Cables that mate from one end to the other can be made.

(This product was my alternative and is in the car but as of now not working right...a classic speed unit. Classic Automotive Innovations / Classic Speed )

Thanks.
Neil
 

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On 102 and 106 cars, the back of the speedo housing has, in addition to the Veglia parts number, a somewhat cryptic marking that describes what an instrument is calibrated for (e.g. "IGIRO=IK", meaning "1 giro = 1 K", i.e. "1 revolution = 1 kilometer" -- and/or something similar for miles). Therefore, the relationship between distance and speed is just a matter of a simple multiplication (i.e. RPMs are in direct correlation to the speed that should be displayed).

One should take into consideration that the first level of calibration is actually for correct odometer reading (accurate distance) which uses a fixed gear ratio internal to the instrument, while the speedo calibration (accurate speed) depends on the strength of the return spring and is a second level of calibration that needs to be done dynamically in context of actual RPMs.

Obviously, accurate reading of speedo/odometer depends very much on differential ratio and tire size (i.e. how does 1 dive shaft revolution relate to the distance traveled, and/or how many drive shaft turns does it take to turn the driven wheels once) as well as on the correct reduction gear ratio from the gearbox to the speedo cable.
 

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Ruedi

Neil is not using an Alfa engine, trans, or rear end. I gather he is using a Speedo drive that uses a GPS as an input for the speedometer, and is asking us about the speedo input characteristics.

Neil - you might chat with Tom Sahines over in the Bay Area. He overhauls these instruments, and calibrates them as well.
 

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Discussion Starter #4
Thank you both. My gps gizmo is not working and I am now trying to interface between a T5 transmission and the original instrument.
 

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So, GPS aside (which is not good at low speed, doesn't work in tunnels and/or streets with many high-rise buildings, and creates difficulties with antenna positioning), there are two conceivable options :

(1) A purely mechanical solution with appropriate gearing and mechanical cable, or

(2) An electronic solution using an encoder (either Hall effect, optical sensor, or non-contacting direction/step encoder) on the drive shaft or gearbox output that feeds an electronic signal either directly or via a microprocessor board (e.g. an Arduino Nano or a $2.00 stm32f103 bluepill) to a stepper motor attached to the instrument. Obviously, the use of a cheap microprocessor would allow for a translation between input and output signals (i.e. programmable input/output ratio) for calibrating the speedo accuracy.
 

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