Monday and Tuesday the highs were 99F on both days.

Monday, I did NOT use sunshade or crack any windows parked in the sun all day at work. On my commute home I got 41mpg.

Tuesday, I DID use a sunshade and DID crack my windows. With my tinted windows it was substantially cooler in the cabin at the end of the day. On my commute home I got 44mpg.

I guess the theory is that a lower cabin temp can help keep a lower battery temp which can leader to better FE.

Anyone else experience this?

 

More likely you didn't have to use the A/C as much on the Tuesday trip.

 

Are you sure that there were no other variables? can you attempt to repeat it a few more times? If it actually is battery temp that would be very interesting, but i would think a higher temp would be give it a better charge (cold batteries usually do not work as well)

 

Higher temp yields higher charge -- perhaps -- (these are not lead-acid or alkaline batteries!) but regardless, high temperature kills battery life, and for warranty protection (minimum 80,000 miles), the software is programmed to USE the batteries in ways that keep them as cool as possible. You can have all the charge in the world and if the computer doesn't like the battery temps, it won't allow the motor to draw on them because high current flow in/out of the batteries generates tons of heat. So when your batteries are really hot, the computer is ensuring they're used as little as possible because the hotter they are, the quicker they die (that is, will no longer accept charge).

cheers --
doug

 

Hum...You could be right. But I think you need to run a few more tests to make sure. Or, you could just use the shade and crack your windows everyday, so when you get in your car it's not as hot!