the speedometers generally read about 5-7 percent high anyway. What happened was the 2.88 differential was put in which compensated for that small discrepancy in your speedometer. after all, 2.88 is 93.8 percent of 3.07 so there you have it - a difference of about 6.2 percent.

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'82 240D 224K miles manual transmission
mods: wooden 4by4 bumper, EGR delete and older EX manifold without EGR port, glass pack muffler (cheapest replacement muffler), rebuilt bosch injectors with Monark nozzles

working on: aux electric fuel pump, coolant/fuel heat exchanger/filter head, afterglow, low oil pressure buzzer/LED