A difference between a dashboard speed display and a phone estimate does not, alone, establish a faulty instrument cluster. The two systems use different information and may update at different rates.
Use a suitable comparison
Record tyre specification changes and whether the dashboard behaviour has recently changed. Do not hold or watch a phone while driving. A workshop can decide what controlled comparison is appropriate and whether vehicle data is consistent with the complaint.
If the display jumps, drops to zero or accompanies ABS warnings, include those symptoms separately. They are different from a stable offset. Avoid coding changes intended to make two displays match without checking the correct vehicle configuration and the requirements of its speed-measurement system.
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- Dashboard Illumination Changes When the Headlights Switch On
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