If your goal is top speed in a sprint, get a good warmup (including a couple 85% sprints). Roll up to just below your lactate threshold (an effort you could hold for an hour if needed) so you're going around 20mph(?). Get your hands in the drops, look ahead, stand, and explode into pedalling. You should be trying to rip your cleats off your soles, bend your cranks, and break your chain. Let the bike swing under you if your form is so inclined.

As you start to spin out, sit down. If you spin out again, lock your ankles at just over 90-degrees. Taking that one motion out of your neuromuscular computation queue will give you another 5rpm -- but it's inefficient and will only help briefly -- enough to get to the line

Gear selection is experimental. It depends on where the bump in your torque curve is, wind, and grade. Some people do best sprinting from 80rpm, some from 120rpm. Some can benefit from a mid-sprint shift, and some can't.