[toy4x4man]

If we really want to get technical on the "thinner air" debate. Warmer air is less dense resulting in less Aerodynamic Drag.

More importantly though, less dense air means that there are less air molecules per given volume being sucked into your engine. Since your MAF and your ECU are constantly trying to match the proper amount of fuel molecules to air molecules for proper combustion if the incoming air is less dense you will use less fuel. Consequently, you will also lose a percentage of power. That is why SAE has exact standards for measuring HP Ratings that include temperature of the air in the intake. You can actually get a good explanantion of this by reading about aircraft piston engines. Also, for those who live in areas like Denver where your air is less dense than someone like me who lives near sea level you will generally see higher MPG's but less power. I can attest to this personally as a good friend of mine that lives in Denver had the same model of Tacoma as I did and no matter how conservatively I drove he always averaged 2-3mpg better than me.

Just figured I'd chime in my knowledge of both aerodynamics and engines.