Sounds like you may have blown the diode to a short circuit in the reverse bias direction, as I intimidated above, although hoped you would get away with it. They are only spec'd to dissipate the tiny amount of energy (back emf) from the collapsing magnetic field of the relay coil when it de-energised, the cause of your speedo jump.
Fortunately they are not too expensive and you have already proven the solution is at hand, so don't be disheartened and just remember 85 is ground. Worth keeping a spare BTW for the others that are OE on your car as a quick plug-in solution.
ETA: posted before I saw similar answer above.
Last edited by Richard Wood; 04/06/20 12:41 PM.
