Obviously this can be dependent on original wheels/tires and which ones you switch to. As I get used to riding my road bike and looking at speeds of others I'm wondering how big of a difference does the gear make?

The first hour of today's ride I averaged right at 18mph with very little wind. The last 35 minutes the wind started to pick up and I had to ride into it for a bit to get back home. Also had a substantial hill toward the end of the ride, all served to bring the average down to a finishing speed of 16.8.

Riding the routes I normally ride, my speeds are usually 17.1 or so with a reasonable amount left in the tank. I could probably average 18 to 18.2 if I pushed fairly hard.

I'm thinking next spring I may upgrade my wheels/tires and see if I can't squeeze another 2-3 tenths of an MPH out of the upgrade. Is that expecting too much?