Huh? I've been cycling for 15 years, many of those years doing 15k km a year, all year round. Mountain biking on mountains at -15C, down to urban cycling in 38-40C scorching summer heat. I've never noticed speed, effort, stamina changing with temperature as you suggest.

And I've done lots of road cycling at constant 35-40km/h, descents of up to 70+km/h.

With N=15 years of gps data and 40-70 of those days per year below zero very few below -10°C. I can say that I ride slower on cold days on avarage.

I am not fit and do not track that at all. I can barely keep my avarage above 25km/h in city traffic for 15km, very few conflict points maybe twenty but not comparable to road cycling. So a km/h decrease of avarage speed during winter is common but not a given. I do not count bad road conditions, but Stockholm has really good maintanance during winter, ice free asphalt all year.

That seems to be born out by using better (read more expensive) winter riding gear. I naturally run a bit cold so spending on low temperature tights, base layer and jersey/jacket has made a massive difference for me.

Scorching heat with high humidity has an opposite effect. H2O drops the density of air and you can go measurably faster than in drier air.

Obviously there could be many reasons for this: less aerodynamic clothing, more cautious cornering etc etc, but the effect is very noticeable to me.

Winter fuel efficiency is worse because of a winter fuel mix, not because of air density.

ICE mpg tends to drop in cold weather, but this isn’t why. It’s usually because it takes longer to reach optimal operating temp or due to changes in fuel composition. Cold air can produce more drag but that problem exists irrespective of ICE or EV.

The pumping efficiency is unaffected as any losses on the exhaust side will be offset by increased pressure on the intake side.

This is why performance cars sport cold air intakes. This is also why your car won’t hit its quoted 0-60 time in a warmer environment — those tests are normally performed or normalized to cold air conditions + 0% humidity. Just what counts as cold depends on manufacturer / reviewer.

Anecdotally, I track efficiency and drive consistent routes, and don’t notice a diff.

Not likely, as an increase in air is going to be accompanied by a corresponding increase in fuel. Unless your fuel mapping is all messed up, it will give you more power, but not likely better fuel economy.

Increased rolling resistance from displacing snow is a far bigger cause of winter driving inefficiency.

That makes absolutely no difference at all.

That would be very surprising if it were true, given the very large effect density altitude has on airplane performance (engine and wing).

Note that it's 3% per 10°C delta so, for a 40°C swing (say 30°C/86°F summer to -10°C/14°F winter), that's a 12% difference in aero drag force.

Opening your window makes far more of a difference.

It's just about observable with very light and very aerodynamic aircraft at ground level if you can get them cold enough.

At -10°C/14°F, with no other adjustments, in the hands of a test pilot, that aircraft will have a ground roll of around 1000' and clear a 50' obstacle and come to a stop in around 1500'.

At 30°C/86°F, with no other adjustments, in the hands of a test pilot, that aircraft will have a ground roll of around 1150' and clear a 50' obstacle and come to a stop in around 1750'.

Flying in and out of 2000'-2500' strips with trees, it's impossible to not notice the performance difference with temperature. Perhaps the people who can't notice those differences don't fly into anything other than very long runways.

Dropping from 18 mpg to 10 mpg would increase your cost of fuel by 80%. It'd be hard to be oblivious to it.