When Helen jogs, every $20$ minutes she checks the total distance she traversed. One day, the first time she checked, Helen had jogged a distance of $2.5$ miles, the second time she checked, she had jogged a total distance of $4.8$ miles, and the third and final time that she checked, Helen found out that she jogged a total distance of $6.8$ miles. What is the difference between her average speeds, in miles per hour, during the first $20$ minutes of jogging and the last $20$ minutes of jogging?