Fix one thing , break another - P1875

[fixkick]

With larger tires on a 98 Sport comes an incorrect speedometer.  

My 'attempt' at recalibration,   I added a reduction box to the Speedo cable.  Now the MPH and Odometer reads very close to perfect.

If this was a Manual trans.. problem solved and done.  But it's not.

After adding the reduction box. Now I throw P1875 codes and eventually a check engine light.

Reason is..  VSS in gauge cluster and Speed sensor in 03-72LE  4 Sp Auto differ by more than 10km/h 


Per the 1800 supplementary service manual for the P1875 code.

DTC DETECTING CONDITION
While driving at higher than specified vehicle
and with more than specified throttle valve opening,
following conditions are satisfied for specified time.
. Vehicle speed sensed by vehicle speed sensor
speedometer) is more than 10 km/h lower than
sensed by transmission vehicle speed sensor when
4WD low switch is OFF, or
• Vehicle speed sensed by vehicle speed sensor (in
speedometer) is more than 10 km/h higher than that
sensed by transmission vehicle speed sensor when
4WD low switch is ON.


Any way of correcting this and keeping my reduction box in the speedo cable and stop the codes from being thrown?     i calculate the difference is now 14-16 km/h

With 40 years of driving under my belt I  would like a correct speedo and not have to do mph calcs in my head while driving or have another
external box to show my speed and  odometer.    I use the odometer for maintenance intervals.

Do I have any options?

The TCM and ECU both boxes in the dash both have VSS sensors. (VSS1 and VSS2)
the taller tires need, VSS1 cable to speed up 10%
the taller tires need VSS2, to speed up 10%'
or you get 1875. (only on sports do we get this code for vss errors

from your post here I see that the so called reduction box is not that, it speed up box.
but no tires told yet to me,
you didnt put those lowrider mexican tires on SPORT did you, I saw four of these folk drive off a cliff one day (death), when all 4 tires exploded at once with radial illegal
load ratings of tires wrong ( super low rated not for a 62 chevy)
why post lacking tires sizes, (load rating is your worry sure)

sure wish I knew all changes on the car, related to drive train (tires too) and ECU and TCM. etc.


the VSS1 is here, rear of box.