A wire off a sensor would light up the check trans or check engine lights.
There are two sensors on the trans, one on the bell housing, another on the tail shaft. You may have pushed water into the sensor wiring connector, time to pull them apart dry them out and clean with a contact cleaner spray . Walmart, Auto Zone ,Napa all have the spray .
May be an explanation for the change but I doubt that it would be the cause of the original 5 mph, discrepancy . Has the tire size been changed, from factory original?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.