Many cars and trucks don't even have a tach. Older Ford focus's don't, they just have a shift light. I used to drive a 70s Ford ranger that had neither. Don't get me wrong I wish it did, but it's only a nice-to-have for regular driving. Shifting off sound is fine, but it's not just sound, it's the vibrations in your seat, how the engine reacts to gas pedal inputs, etc. I only look at the tach when I need to downshift to pass.