I have noticed the same. Chemistry tells us that air is more dense at cooler temperatures which would increase resistance. This may or may not be a measurable factor.

Could be especially at multisport pace. Air is more dense but sometimes you can make more power too and it about cancels out. Could also just be the direction of the wind or other things

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

I find this to be the case.

When I ride in the winter, my average speeds are always a lot slower.
This also could be due to being less fit (in my example); but maybe the cold temps make the rubber in the tires harder, increasing RR?
In addition to the air being denser.

Even roll-downs that I normally average a given speed in warmer weather, are *always* slower when it's cold out.

So, IMO, lower temps = slower temps.

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float , hammer , and jog

I had a very similar experience this weekend as well. I had a higher power output but my average speed was low and my time was longer than other races of the same distance. It was 7C (44F) during the bike leg and wet to add insult to injury. I have been scratching my head as to why I was 9 minutes slower than I had anticipated and this could be an explanation.

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"Train so you have no regrets @ the finish line"

Doesn't this depend on whether your body can handle racing better in heat or cold? Personally, I race ride faster in colder weather...I prefer 40s, 50s, and 60s, vs. 70, 80, or 90s.

Tires roll slower at colder temps as well :-)

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http://bikeblather.blogspot.com/

Yes, people will be affected by heat differently. Could depend on race pace too. If you are up near threshold (oly pace) and really powerful (due to talent or size) heat is more likely to limit you than ironman pace or someone smaller.

I tend to make more power in the cold but speed doesn't change. I cancel out!

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

This is a known fact, yes.

Drag is 1/2(density)(Velocity squared)(CdA)

Temperature affects air density, which affects drag. The colder, the more drag. You can find charts for density of air vs temperature to find out how much. it's not much, perhaps a 4-5% difference in the density value, so if you apply that linearly you should have 4-5% more drag. Same goes for air pressure, which will affect air density. this is one of the reasons why specific locations, dates and times are chosen for things like land speed record/hour record attempts.

All else being equal, you will bike faster on a hot day than on a cold day. but of course other factors, like body acclimitisation also play a big part. Too hot and your body suffers.

What Jack said, can potentially push a bit more power (than a really hot day but not much more than a mild day) but the air is more dense. More likely culprit is combo of air density, worse roads and specific wind conditions.

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Founder: BestBikeSplit
Amazonian

And possibly different clothing for cold weather.

This calculator has an entry for temperature as one of the variables. A quick hypothetical for me gives a .2 mph slower speed at 250 watts when the temp drops from 70 to 50. And I assume that does not take into account any extra clothing one might be wearing.

http://www.kreuzotter.de/english/espeed.htm

Some of my best cycling has been on hot days. Pay for it on the run, though.

Tires roll slower at colder temps as well :-)

Yes, not a negligible effect.

I'm a lot slower at cold temperatures, but this may have to do with the 9 layers I put because I hate cold weather so much ;-)