Below is a comparison of the first half of my rides the last two weeks. I only took the first half because the route is fairly flat and doesnt have too many stoplights. So here goes...
Date: 10/5/08 Pnorm = 214 W Pavg = 205 W Avg Speed = 21.7 mph
Date: 10/12/08 Pnorm = 217 W Pavg = 207 W Avg Speed = 20.5 mph
As you can see, although the power values for both rides were about the same, the average speed on 10/5 was 1.2 mph higher. The main difference? It was really windy yesterday whereas the 5th was a relatively calm day. Of course everyone knows that a headwind or crosswind will slow you down. The problem is that it is hard to say how much. Additionally, because struggling to pedal along at 12 mph into a headwind feels so different than flying along at 30 mph with a tailwind, it is often hard to use PE to determine which is better workout.
This just another advantage to owning a power meter. Unlike speed, which is highly dependent on external environmental variables, power is only affected by how hard you are working. So whether you are going 12 or 30 mph the power meter doesnt care.
The map below encompasses most of the first part of the ride but I have to admit I dont know all the road names so I only made it as far as I know ;)