I did 19.2 miles today.

I think my old computer was badly prone to interference. I'm averaging around 14.5mph (stops and the mile-long incline included) with this one. With the old one, I'd never get above an average of 12.5...and I was using this thing from the week after I got the bike until about November, when I discovered it wasn't actually registering anything. If this is the case, this explains why I hadn't seen any improvement speed-wise!