I think the point several folks made about all the variables involved is extremely valid. I did a 30 mile ride on Monday on my ~30# ATB with slicks and averaged 16.1 mph. There was a little climbing on some very mild grades, but there was also a nice amount of downhill. I had a headwind half of the time, and tailwind half the time. But I could not compare myself to someone else on a similar distance, if their terrain was very hilly or very flat with a lot of wind or without. I think the only time average speed really matters is if you have riders competing (for fun or for real competition) to see who is faster.

What is the most important thing is that you have fun.

And as far as the OPs question, I think most riders on here are pretty honest as far as avg speed, what they may be omitting is the factors of their ride that can contribute or detract from it.