calculating avg speed

I just did a quick 2.5 miles. On the way out, I kept between 15.5 and 16. On the way back with head winds I kept between 12.5 and 13.5. My puter reads avg speed 11.9! I know that I have the wheel size right. I did have to stop at a light once out and once back and it was a really long light. Does that account for my low avg? There just isn't anywhere out my front door that I can ride more than 1/4 mile with out a stop light or sign. It makes it a little tough to do intervals. Get up to speed, ride hard for most of a block, slow down, stop at sign, repeat.

Does your computer have an auto start/stop feature? If so, be sure it is set to it so it doesn't keep calculating time. Even so, just walking the bike from your door to the sidewalk with affect the time.

This is a pretty basic model. I don't think it has auto start/stop but I'll check it.