Its an interesting one - fuel economy in colder conditions. Heavier fuel consumption from cold with the colder conditions, better air intake but also higher density air to push through. On a recent drive to Sydney and back in 'relatively' cold weather I noticed two things.
1. Our fuel economy was worse than expected (MFD 7.8 way down, 7.3 way back, being 8.0 and 7.5 real as measured). I would have expected an MFD reading more in the high 6's very low 7's.
2. That I noticed wind noise from our roof bars and roof box far more than normal (and it wasn't excesively windy)
1. could be because we had the roof box on, though I have achieved better fuel economy on similar trips with the roof box on in warmer conditions. It is also possible that the roof box was not aligned with the car as well as it should - I checked, possibly off a bit but not obvious.
I have noticed in other cars that Aerials will whistle when conditions are colder and I have noticed our roof bars are noiser now in colder conditions - I presume from the air being more dense?
I did a search on the net, and contrary to what you might think air density is actually lower when humidity is higher. So hot humid air, is significantly less dense than cold dry air. This is due a law of physics observed by Avogadro, see
USATODAY.com for a fuller explanation. Conditions in summer up here are generally hot and humid.
Higher air densities are probably most relevant at high speeds, but its interesting to note that even at around 110klm/hr, a Tig is pushing through 1 tonne of air every 7 seconds, or almost 5 tonnes every klm travelled (av air density is 1.2 kg/m3, and assumign frontal area of 2.2*1.8m). So air density may well affect fuel economy, just as wind direction (head or tail) will. Colder conditions may not equate to better fuel economy, though I dont know the balance b/w upside of higher density air going into engine versus car pushing through higher density air to get anywhere, especially at highway speeds.
