According to the Washington Post, the widest tornado ever recorded was the El Reno tornado that touched down west of Oklahoma City on May 31, 2013. The twister reached a maximum width of 2.6 miles.
According to the Washington Post, the width of the El Reno tornado was equivalent to the entire length of New York City's Central Park from north to south. This tornado was rated as an F-5, the strongest category available, and two graduate students from the University of Oklahoma recorded a wind gust of 296 miles per hour, the second highest wind gust ever recorded on the planet. It missed the record by a mere 5 miles per hour.