If 185 is the width of the tire in millimeters that's a very very narrow tire that will easily be overpowered unless you have a small engine (as in well under 3 liters) that does not produce much torque/HP. I don't think he misunderstood the size, in North America tire sizes are rated in "mm width/aspect ratio-wheel diam in inches". For example the rear tires on my 1970 Duster are 275/40-18.
Anyway to answer your original question, like @MOPAROFFICIAL said torque converter stall speeds are rated at the (expected) maximum input torque. Below that torque, say cruising or taking off at part throttle the stall speed (more correctly, difference in input vs. output RPM) will be lower. Also since your 42RH has a lockup converter, in most cases when cruising the torque converter will be locked so there will be no slippage or difference in input and output RPM from the converter at all.