Alfano 6 GPS vs Magnetic speed inconsistency

Does anybody else running an Alfano 6 or even a Mychron or any other data logger suffer from inconsistencies between GPS speed and magnetic wheel speed:

The delta chart at the bottom is based off the GPS speed and distance so when the rear wheel magnetic speed is telling something else it can get a bit confusing to analyze laps.

Is the bottom plot = GPS speed - Mag Sensor Speed? Is the discrepancy alway at the same point on the track or does it drift around? I used to run an the older GPS with a MyChron 4 and it was terrible, sometimes it would be off by .25 seconds when compared to the track timer and the speed traces would not line up from one day to the other, so for the past 7 years I run a M4 and the wheel speed sensor. Interestingly, Ryan Norberg still runs the M4 and wheel speed but hardly no one else does.

As I understand, GPS relies on satellites and they move around, which cause drifting errors in the calculation of lap time and maybe speed.

Are the M5 more accurate with the lap times?

Top plot GPS bottom plot magnetic.

It seems to mainly be around mid-corner.

At the track it normally has around 15-25 satellites connected so I’d have thought it wouldn’t drift too much.

Does running your M4 with just a mag speed still give you a delta time?

Based on what you said, I would check for lateral play in the rear axle vs. the clearance on the sensor. Shifting or deflection can cause read errors, which would cause the large peaks and valleys.

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Yes, I get delta time on the M4 with speed only. I also have an Expansion Box connected to the M4 but I need that only to create track maps. Today, track maps are available online for most of the asphalt tracks.