Tornadoes travel a very narrow path, hardly ever tracking directly over a weather station that can measure wind speeds. Furthermore, many wind instruments couldn't survive the intense winds in the core of a tornado.
So meteorologists wait until after the tornado passes, and then send out a storm survey team to analyze the damage the tornado caused. Through a survey of the damage to homes, trees, cars, and property, meteorologists will then estimate how fast the winds likely were using the Enhanced Fujita Scale, which ranks tornadoes on a scale from 0 to 5 based on intensity.
An EF-0 tornado can have winds from 65 to 85 miles-per-hour, while an EF-5 tornado can have winds over 200 miles-per-hour.