Hi,
ECU and instrument cluster uses the same sensor, but calculate the vehicle speed separately.
The instrument cluster gives an very inaccurate analog result.
If you calculates AIMVM correctly, the ECU will calculates the vehicle speed with great precision.
The Wheel speed sensor gives 8 impulses every full revolution of the wheel.
To calculate AIMVM exactly, you must divide 8 through the right wheel circumference.
As example:
AIMVM = 8 : circumference [m]
Rolling circumference of 255/35_18 = 1,99711m => AIMVM = 4,00579
Rolling circumference of 235/35_19 = 2,032925m => AIMVM = 3,93522
***
An other way is to calibrate the instrument cluster.
This is not testet by me yet, but I think it is possible to recalculate the MAP of the analog speedometer.
Read out the 93c86 EEPROM of the instrument cluster and you will see some interesting MAPs.
I think there are MAPs for the instruments between address 1AB and 247, and between 6AC and 6E7.
No idea whether there are checksums, but in the area of stored odometer value at 042 - 051 there is no checksum activ.
This part is testet by me.
In the dump you will also find the VIN and the IMO number, which will be compared with the VIN and IMO data in the EEPROM of the ECU.
Here a hexdump of my RS4 VDO instrument cluster EEPROM.
