Severely Erratic Speedo Reading

[essb00]

DI 2 assigned to GP Speed 1, then GP Speed 1 assigned to both Driven & Driving Wheel Speed.

I'm getting severe fluctuations on the speed readings - reaching 5000+ km/hr. 
In between the big fluctuations, smaller ones I can probably filter by using Math function (average) - but the big fluctuations would still affect the computed average.

Is there a way to filter ridiculously high frequencies being picked up by the DI? I don't have an oscilloscope to confirm at what frequencies the big fluctuations come at. I tried grounding the speed sensor on different locations. I even used shielded cable which made no difference.

On the log, I'm using a reed switch speed sensor from gauge cluster (speedo gauge driven by cable). I also have tried to use a magnetic/hall effect sensor with magnets placed on propeller shaft. Both of them pick up the same high frequency noise.

Tune:
https://drive.google.com/file/d/1PdrflxK3JTEZHSuwEj50PTJNHILF5YVi/view?usp=sharing

Log:
https://drive.google.com/file/d/1BBte-Og8anpc4ToLCXAq1oULVYd5c1zA/view?usp=sharing
*vehicle on jack stands - reason why TPS % seems low. 

Maybe I'll try to make a simple low pass filter circuit with parts I got at home.

[essb00]

I have made a passive RC low pass filter (diagram below) for the GP Speed 1 input, made a math block av (as GP Speed 2), and had been assigned to both Driven & Driving Wheel Speed. Wheel speed readings now seem useable - I just need to find the correct calibration (I have to compare by GPS).

Tune:
https://drive.google.com/file/d/1CIfhH23qxXs6FufS1ifCAT2hXVko3ZIU/view?usp=sharing

Log
https://drive.google.com/file/d/1_HutvefwpZH6lMlfT20M7qJPg71ZHptw/view?usp=sharing

 

@Adamw I know I read somewhere before where you mentioned the DI falling voltage to be considered by the ECU, but I can't seem to find it anymore. Should it falling below 1.0v or 1.5v?
I just want to make sure so I can try to make fine adjustments on the frequency cut-off by changing the values of the resistor or capacitor.