If you want to be nice to your knees, don't push too big a gear. Certainly don't push too big a gear when standing. Lower gears with higher cadences are kind to knees. And when standing, choose a gear that allows you to stay on top of it, so you have the speed to get through the dead spot at the top of your pedal stroke before applying too much pressure. You want to feel as if you're dancing on the pedals.