Yeah, sounds about right to me.
Like you, I've been doing the same commute for the past number of years. My light road bikes are faster than my heavier road bikes, but not as much as you might think. For comparison, an 17-lb carbon road bike with 25mm tires gets me to work about 2-3 kph faster than my 28-lb steel road bike with 38mm tires.
Here's a run-down:
Take the 17-lb road bike as the benchmark:
19-lb aluminum bike with 28mm tires, no difference
24-lb aluminum cx bike with 32mm tires, -1 kph
24-lb aluminum cx bike with 35mm studded tires, -7 kph
28-lb steel cx bike with 38mm tires, -2 kph
With the number of stops and lights on the route, it brings the average speeds closer together. If, however, it were a continuous ride on rural roads with very few stops, the difference with be much bigger. This is why I commute with the last two bikes on the list for most of the year. The bigger tires are more forgiving during the winter months, and can haul a lot of gear with racks and panniers.
Last edited by mcours2006; 03-09-21 at 07:33 AM.