It all depends on your weight and height and of course how fast you are but an average time for a 40 yard dash is 6.2 s ( or 6.7-7.3s) if you've heard 5.5 s than you're mistaken. That's definitely not an average time , that's more like a pretty fast time.
At age 11 in New Caney Independence School District's 6th Grade, (In New Caney Texas) Recorded in the spring of 1978, Where had the record for the 50 yard dash; My best time in 50 Yard Dash is 5.23 seconds
40 yard dash/10 yard dash= 4 What ever the time you got for the ten yard, you multiple it by 4. It could be an average of your running time per 10 yard dash. Sometimes for several runners, they would feel worn out by the time when reaching the 40 yard mark.