MPH

I must increase my average miles per hour, I feel like I'm going fast on average, when I look down at my computer I am going 17-18 but then my average is about 15. What's the deal?! I rode in the gym today because the weather was rotten, I rode for 60 minutes and went 17.44 miles. I don't feel like I pushed it or went any harder then usual. So what does this mean? I came to the conclusion that riding in the city or anywhere with lights, stop signs, traffic, etc, greatly kills my average mph. I am going to try and do a few rides with very minimal stopping and see if that ups my average. I have to average 17 mph to make the STP in one day under 12 hours. I'm not sure I want to do it in one day, but I am shooting for the moon here.