Based on actual measured studies i believe the average to be somewhere between 5.16 and 5.6 inches. I put zero weight on self reports. I forgot the website, but there is a site that takes pictures of erections on a standardized background and they do measurements. One example stuck out in my mind. It was a penis around 6 inches measured. The man self reported his penis as 8 inches.
The largest result from an actual measured study I've seen is the Lifestyles study. That study came up with an average of 5.8 inches but I think it suffers from selection bias. They measured sexually active college studies on spring break in Cancun. You are not going to get many guys on the smaller end of average to volunteer to have their dicks measured by nurses while they are on vacation, whereas the bigger guys will line up at the chance.
It seems like the bigger the study the closer the average gets to 5 than it does 6 inches. As far as I know, there has never been a measure study that had an average result of 6 inches or more. You only see averages that high in self reported studies.