Just had this discussion during a small group ride Saturday. Over the same relatively flat 30+ route, I consistently average 14 to 14.5 mph on my Specialized Sirrus Comp. Over a 35-mile course with a steady 2-3 percent grade, and two lengthy 5+ grade hills, I average about 13 -- trying to keep my pace above 10 on the steady grade, above 6 over the climbs, and flying back home.
Heat also had an impact -- below 80 degrees I'll consistently avg. about a mile per hour faster.
It's also interesting that on a 64 mile ride this spring, I rode with a couple of good roadies and avg. 16.5 over the first 40 miles but paid for it on the last leg -- ended with a 12.6 avg. speed. Same course on my own pace a month later, avg. 14 mph. Don't know if this is because I stayed within my comfort zone throughout the ride, or I was just in better shape ;-)
And don't ask my pace when I'm just out for a fun ride with lots of gawking!