I think hills lower your speed at least 2mph compared to a shorter flatter commute, more if you're like me and run out of steam pretty quickly. I was happy with 16.5 mph last weekend, 30 miles on North Georgia hills. If I went 40 or 50 it would creep down to closer to 15. In town, shorter and flatter routes that I'm accustomed to, 3-4 mph faster. I think it's impossible to accurately compare speeds with someone else unless they're covering the same ground at the same time (ie, riding with you).