Help me make sense of E46 AKMB Speedo / Tacho coding

[bimmerdriver]

I'm doing a project on my E46 M3. I want to correct the speedometer, which is configured from the factory to read higher than actual speed. I picked up a bmw scanner so I can get at the EEPROM to fix the place where the value of the displayed speed is inflated. However, while I was at it, I wanted to make sure the speedometer is set for the correct number of pulses corresponding to the size of tires on the car, which are not OE sizes. (I'm running a 285/30-19 rear tire.) When I looked at the coding of the AKMB module, I found something that doesn't make any sense, namely these values, which are set for much larger tires than anyone could even put on this car. A screen capture of ncs dummy is attached to show the optional values.

K_ZAHL_TACHO
	4846_imp/km
K_ZAHL_TACHO_KOMPL
	4846_imp/km
K_ZAHL_WEG
	4797_imp/km
K_ZAHL_WEG_KOMPL
	4797_imp/km

The OE rear tire sizes are 255/40-18 or 255/35-19 and the CSL had 265/30-19. The first two are 496 revs/km and the third is 510 revs/km. 491 corresponds to a 265/35-19, which is a "taller" than standard tire. I don't understand why these values would be in the daten, unless they were modified by someone. When I bought the car, it was wearing 265/35-19, which could explain the 4912 value. Shouldn't the values for the speedo and odometer be the same, and correspond to the actual tire sizes?

[RocketSurgeon]

Those values appear to be factory. The vehicle determines speed by reading impulses from the ABS wheel speed sensors. The sensors generate many pulses for each revolution of the tire so those impulse/km values look similar to other vehicles I have coded.
The odometer will be coded to be quite accurate, though the speedometer is coded to be somewhat optimistic. I would start by making sure the cluster correction factor is coded out, then test speedometer accuracy against GPS. Then make corrections from there. You could input custom values through nettodat if needed.

[bimmerdriver]

Those values appear to be factory. The vehicle determines speed by reading impulses from the ABS wheel speed sensors. The sensors generate many pulses for each revolution of the tire so those impulse/km values look similar to other vehicles I have coded.
The odometer will be coded to be quite accurate, though the speedometer is coded to be somewhat optimistic. I would start by making sure the cluster correction factor is coded out, then test speedometer accuracy against GPS. Then make corrections from there. You could input custom values through nettodat if needed.

Thanks for your reply. This is interesting. As far as I can determine, the ABS reluctor ring on my car has 48 slots. Does a 48 slot reluctor generate 48x2=96 impulses per revolution? I'm not sure how to get from ABS impulses to the values. The revs/km of the standard tire is 496. If there are 96 impulses / rev, then the impulses / km should be 47616, which are 10 times larger.

[bimmerdriver]

Those values appear to be factory. The vehicle determines speed by reading impulses from the ABS wheel speed sensors. The sensors generate many pulses for each revolution of the tire so those impulse/km values look similar to other vehicles I have coded.
The odometer will be coded to be quite accurate, though the speedometer is coded to be somewhat optimistic. I would start by making sure the cluster correction factor is coded out, then test speedometer accuracy against GPS. Then make corrections from there. You could input custom values through nettodat if needed.

Not sure how it was determined, but apparently either the DSC or the cluster software takes the 48 pulses / rev from the ABS reluctor and converts it into 10 pulses /rev. Based on that, the values for the tires in the odometer are way off. 4961 corresponds to both the 18" and 19" standard tires that the the M3 was equipped with. So this means the odometer is reading 1% or over 3% higher than it should, if the standard tires are fitted. If you have shorter tires fitted, the error is worse. In my case, the errors are either over 2% or over 4%. This is really strange. I understand German law requires the speedometer to read high and it's understood that the cluster is inflating the analog reading relative to the digital reading, but I don't understand why the odometer should be configured to read high, except to run the car through the warranty period faster.

[RocketSurgeon]

Not sure how it was determined, but apparently either the DSC or the cluster software takes the 48 pulses / rev from the ABS reluctor and converts it into 10 pulses /rev. Based on that, the values for the tires in the odometer are way off. 4961 corresponds to both the 18" and 19" standard tires that the the M3 was equipped with. So this means the odometer is reading 1% or over 3% higher than it should, if the standard tires are fitted. If you have shorter tires fitted, the error is worse. In my case, the errors are either over 2% or over 4%. This is really strange. I understand German law requires the speedometer to read high and it's understood that the cluster is inflating the analog reading relative to the digital reading, but I don't understand why the odometer should be configured to read high, except to run the car through the warranty period faster.

Keep in mind that your spec is for a brand new tire with the full 10/32nds of tread. As the tread wears away, the tire spins more revs per kilometer. If your tire spins 496revs/km when new, it will spin 506revs/km when worn down to the legal minimum of 2/32nds.

[bimmerdriver]

Keep in mind that your spec is for a brand new tire with the full 10/32nds of tread. As the tread wears away, the tire spins more revs per kilometer. If your tire spins 496revs/km when new, it will spin 506revs/km when worn down to the legal minimum of 2/32nds.

Yes, understood. That makes the already optimistic reading even more optimistic.