I don't think you have a lockup converter, but there is little converter slippage at highway speed on a level road probably no more than two percent. Also the RPM/tach relationship is fixed and will not vary with installed tire size, so use the OE revs per mile in your calculations.

Shorter tires will increase speedometer error, but will not effect the speedo/tach ratio. That is fixed by the speedometer gearing.

Duke