How much slower should I expect to be in winter rides?

Question

I typically do ~100km rides (almost always alone). In summer, I manage average speeds of 30km/h or more. This is the first (of only three in total anyhow) winter in which I keep riding somewhat regularly in winter.
I did > 1000 km per month in the summer months, and around 700km (some of which indoors) in the winter months (but extra running etc.). So I would expect my form and fitness to be somewhat lower, but not dramatically so. For instance, average watts on the indoor trainer as well as times when going running are fairly constant. (My PM also shows fairly constant wattage on the bike.)
Yet, average speeds are more like 27-28 km/h at the moment.
(In my case, although I hope the question could be of more general interest, I also ride a new bike which I had hoped would be faster although it is an endurance bike, simply because it is more high level in terms of the frame than my previous fairly entry level alu one.)
My question: Is that an expected decrease in speed? If so, is it possible to break down this difference into different components (next to, possibly, less fitness)? I think of

typically more windy conditions, which hurts average speed
more layers of and less aerodynamic clothes
wider tyres (28mm insted of 25mm for me, also lower quality ones; also, the endurance bike maybe has somewhat less agressive geometry)
more rolling resistance on wetter surfaces
more careful riding style on wet surfaces (surely hard to quantify, but definitely applies to me)
I heard something about colder air implying more resistance
more energy needed to keep the body warm (although I rather overdress, I think)
psychology?
anything else?

Kibbee · Accepted Answer

This source goes into quite a bite of detail on the subject.
The three main reasons they go into are higher air resistance in the cold because cold air is more dense.  Increased rolling resistance as cold tires cannot deform as much as warm ones. They also mention the bulkier clothing causing more air resistance.
One advantage of colder weather is that you are less likely to overheat. This can increase performance.  I have felt this effect of being able to maintain my efforts longer on cool fall days, while cycling on my trainer is often more difficult as there is no air flow to carry the heat away.  Also, cycling on hot summer days can also be exhausting.
Just be careful not to overdress for the cold as having too many layers could also  make you overheat. It's ok to start off cold as you will warm up during the ride.

Michael · Answer

I think you’ve listed all the possible reasons. Though I have one more detail regarding the body and cold: From personal experience it at least feels like blood flow is worse when you are cold.
You’d need a power meter to find out if it’s actually your power output which is lower or if it’s your equipment, road conditions etc.

juhist · Answer

I have created a simulator for cyclist speed. Some results from that:

Continental Grand Prix 5000, the best overall tire, gives 28.5 km/h speed whereas Continental UltraGatorSkin gives only 26.3 km/h, at the same rider effort. Thus, variations in performance of slick tires alone can account for 2.2 km/h. If one of the tires has a tread pattern, the difference would be larger.

Air density has a smaller effect. At 20 degrees Celsius, performance is 28.5 km/h whereas at 5 degrees Celsius, performance is 28.0 km/h.

Wind and less aerodynamic clothes could also explain some of the difference, but I think if you are using a different endurance frame geometry for winter, the difference from the frame geometry alone would be large.
Try sometime using your summer bike in the winter, with the summer tires and more aggressive frame geometry. As long as there is no snow or ice on the roads, it's perfectly safe and would tell you whether the difference is in clothes/wind or in the bike.

Chris H · Answer

I've noticed the same, and largely blame the wind and tyres (same bike, but I've  changed my tyres for ones that better handle mud on the road).
However there are 2 closely related factors that may  play a part: nutrition and hydration.
Nutrition first: You're quicker than me, and I don't stop to eat so I assume you don't either. In winter gloves it's harder to get out and open packets while riding. This is even true for opening gels with one hand and teeth. The result is that delaying a feed until a good bit of road  is more likely (compounded by puddles, fallen leaves, darkness etc. making some bits of road worse).  I may end up having half an energy bar because I stuff it back in my pocket/toptube bag to free up my hands when I see a poor surface coming up, then don't go back to it.  In fingerless gloves I'd be more likely to hold the snack on the bars.
Hydration: I always drink as much as I want and riding conditions allow, and the latter may be affected by winter conditions similarly to food.  But to some extent I feel like cold suppresses thirst, or at least counteracts it.  This may be because of vasoconstriction leading to a full bladder, but still needing to drink, or it may be because consuming large quantities of cold water when I'm only just warm enough lacks appeal, so I have smaller drinks. Much of your water intake  is needed for digestion, and the hills are still sweat-inducing on the way up.

How much slower should I expect to be in winter rides?

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