For roughly 2/3 of the year, maybe slightly less I commute on an aluminum cross bike with 28 mm tires. The rest of the year I commute to work on a carbon fiber bike with 23 mm tires. I like both bikes but the carbon fiber bike does cut the commute from roughly 42 min to about 37 min. Sometimes I'm faster and sometimes slower, but usually the carbon bike is about 5 - 8 min faster.

One thing that I find interesting is that on days when I'm on the carbon bike and riding along at say 25 km/hr that guys will blow by me and try to race me. Sometimes if I feel like it I'll up the pace, chase and see what they have left in the tank. Other times I just go my speed and perhaps raise my speed to say 28 to 30 km/hr. Most times I'll eventually catch the guy who passed me.

I don't get this on the aluminum bike. I wear the same clothing on both bikes, but if I ride the carbon and wear a team jersey like Saxo Bank or HTC Columbia it's almost guaranteed I'll have someone try to race me. Most of the time I can resist the bait, but often I cannot.