Their are just to many variables to answer your question. The rate at which your torque converter will lock-up all depends on the throttle position, RPM of engine, vehicle speed and load on the engine. I do not remember the characteristics of the stock trany, it has been to long. But generally my ATS trany will want to go into lock-up as soon as possible. This means with just the truck and no load, I will hit lock-up as low as 35 MPH. Normal highway entry ramp will be 55 MPH. Like I said, there are just to many variables.