56kz (and formerly, TJ) is right. You drive by feel more than by numbers.

On a related topic, I often make use of the OD off button on my auto when climbing steep grades. Rather than gradually pushing down more and more on the gas pedal until the TC decides to unlock and then until the tranny decides to drop from 4 to 3, I just go ahead and get those revs up early without mashing the gas to do so. Similar to how I would anticipate a hill when driving with a stick.