I read an article that said that cars don't go as well in hot air because the air is less dense. Now I know that air resistance is directly proportional to the density of the air, which would mean they would go better in hot air. It turns out that the article is talking about engine performance. If the air is less dense, a given volume contains less oxygen and the engine might thus be prone to incomplete combustion. I can find no formula relating engine performance to air density but there's an explanation of the issue here.
