The relationship between speedometer and odometer
 

Dear friends:

I have never removed an instrument panel to look at the actual instruments in a car. I'd like to know whether an odometer actually gets its numbers based on the average speed fed to it by the speedometer (i.e. a single signal cable for the speedometer from a speedometer sensor), or a speedometer and odometer get their numbers from 2 separate signal cables coming from 2 different sensors? Thanks.

Best regards,

Eric

Eric:
A non-electric odometer "gets its numbers" from the same cable that drives the speedo. The number of teeth in the gears that drive the cable, move the odometer etc. are related to the OE tire size and thus you get a proper reading. When you change to significantly larger tires, as on a Jeep, you then must replace the gear on the tranny side of the cable to account for the difference in travel per rotation, or your reading of both speed and distance will be off (low). Now if your talking about electronic speedo's (don't know what kind of car you are referencing but your language leads me to believe you are), I do not know for sure but I would assume it is a speed/time calculation made by a computer, as most of those cars can use the same calculations to give you range, average mpg, average speed, etc.. HTH

Dear bjcsc:

I was thinking about W116 300SD (1978-1980) and W126 300SD (1981-1985). I heard that W116 300SD has a mechanical speedometer and W126 300SD has an electrical speedometer. I believe that in both cases, the speedometer (mechanical or electrical) drives the odometer via gears.

For a LCD-based odometers, it's pretty sure that a tiny chip handles the calculations based on data from the speedometer.

Thanks again,

Eric

Hmmm...so basically you already knew everything I wrote? If so, sorry. I would agree with you because I think that even if the speedo is electrical, the signal is probably still driven by a cable and thus the odometer driven the same way as if the speedo were mechanical. However, if the 126 gets a signal from a wire, I too would wonder how the odometer works. Which I guess brings us back to your original question! I'm a lot of help, huh!:D

While on that topic, isn't it interesting that although the same info (mechanical or electrical) is fed to the speedo and odo, that the odo on earlier cars sometimes fail, while the speedo keeps on chugging?

One of the car mags recently ran an article concerning odos and speedos. They reported that German regs dictate that speedo error MUST be on the high side, and can be within a certin allowance. However, it also mentioned that late model cars have no connection between their odo and speedo. The odo must be highly accurate, and it's error is not a function of the speedo error.

There are actually three different types of speedo/odo. Mechanucall driven, electronically driven and digital.

The first and second types have a single input and that is converted to two different displays (actually digital does this also but differently.

The mechanical uses a cable which spins a magnetic speed disc inside the speed cup/drum that is attached to the needle. The odo like any straight counter is just geared off of it.

The electronic takes a pulsed signal and separately creats the speed and odo displays. The odo display is rotated from a motor through gears.

The digital gets its input usually from the ABS via CAN and calculates and displays speed and odo readings separately.

Dear friends:

Thank you all, especially Mr. stevebfl, for your helpful information.
BTW, what does "CAN" stand for?

Best regards,

Eric

Controller Area Network. That is probably a copywrited acronym as it is Bosch's nomenclature for their specific network protocol.

In other words speed is interpreted by the traction control, viewing wheel speed sensors and transfered to whoever needs such a signal digitally over the CAN. To the instrument cluster in this case. The speed is indicated by a meter movement as is the gas gauge.