AC on the entire time and I got my best MPG average ever??

[azraelswrd]

Warmer air is less dense, so that could be a factor or with the extra ambient heat your engine was able to operate more efficiently sooner. I don't know... I'm a bio geek by trade.

But this topic renews hope that I can use my A/C some more. I live in Central California and like San Diego we're cooking like there's no tomorrow. Oh lookie, its supposed to be only 104F today! That's a relief... that cooler than yesterday.

[MechEngVT]

Could you tell if your engine was running any hotter? Typically A/C condensers are mounted in front of the engine radiator and therefore your radiator is drawing pre-warmed air to cool the engine coolant. Combine that with the already high outside temperatures your engine may have been running hotter.

Engines that run hotter are theoretically more efficient because they come closer to approximating the Otto cycle ideal adiabatic process where no heat is lost during compression of the intake charge. Hotter cylinder wall/head temperatures mean less compression and combustion heat is lost to the coolant and more is converted to work. Modern vehicles are tuned around 205 degree thermostats because this temperature has been found to be more efficient than the 160-180 'stats commonplace 20 years ago. You only run into problems with higher heat if detonation occurs (especially if your ECU pulls back timing advance due to knock sensors avoiding knock conditions) then mileage decreases and damage can occur.

Humidity (relative humidity) can have a large effect on engine performance, possibly as much or more than temperature alone. My truck seems to like cooler temps and higher humidity so it's probably pulling in some timing to avoid knock on 87 in hotter/dryer conditions.

[digitaldissent]

hey there I am in SD too and yeah its been HOT, I was stuck in more traffic than normal and using the AC the whole time. I kept thinking this tank is going to suck but then I filled up and still got 40!

has to be the hot dry weather or I am just gettin good with the hypermiling