VDO SUMLOG Manual page 11

Press the pushbutton to increment up to the calculated calibration value.
Each press increments the value by 0.01. Incrementing is automatic if
you keep the pushbutton pressed.
The calibration factor setting range is from 0.50 to 2.50. On reaching
2.50 the counter rolls over to the calibration factor 0.50 again.
The set calibration factor is stored if the pushbutton is not activated for a
further 5 seconds. The display then automatically changes to display the
total distance.
The calibration factor also applies to the speed indication.
In the case of a measuring run in flowing water repeat the same distance
measuring steps in the opposite direction (measuring length C-D).
The calibration factor is calculated with the following formula:
Effectively covered distance (A-B) + (C-D)
C = ------------------------------------------------------------
Indication of the display (A-B) + (C-D)
Do not use the GPS navigator as a reference for VDO
Sumlog calibration. The GPS Navigator indicates speed
over ground (SOG), but the VDO Sumlog measures speed
through water.
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CALIBRATION