Why do warmer air temperature's increase MPG?

When outside temps were in the 80's and my original Integrity's were close to their wear limits, I was easily seeing 55mpg on average during my commute at 65-70mph and 36 miles each way with a best at 59.3mpg.

Since I've replaced my tires with new Integritys which only have a few thousand miles on them, and since temps are now avg 108-115F outside, and the air is running strong, my mpg is back down to 45-48mpg average. While we have a dry heat (6% humidity recently), we still need A/C to keep things reasonable.

The two factors (not sure which is stronger) combined seemed to have made a significant hit on my mpg. While rolling resistance and air density may be less, the need for full blast A/C (even when set at 78F) seems to counter any benefit.

As I type this, however, I"m in SoCal for the weekend, and I made a trip down to San Pedro (LA Harbor), My trip average there was well over 50mpg in the cooler temps. Drive back wasn't as good due to more hills, and the addition of CA gas to my tank (made it from Surprise to San Pedro - 405 miles +/- on one tank, not bad).

Other factors are that I need to re-check my tire pressure, and I'm probably due for an alignment since my tires are wearing unevenly (shimmy/vibration in the wheel during turns that coencides with wheel rotation) - no thanks to a couple of rolled curb to flat curb transitions I keep seeming to nail while parallel parking.

Once I get my pressures re-checked and my alignment eventually done, and once my tires have broken in a bit more, I'll be able to verify whether or not higher temps are the only true factor in my recent MPG drop.

 

Update:

Drive home from averaged 48mpg.

Checked pressure this morning - hot, they were at 35.5 average all the way around. I bumped them to 43/41 (hot) and my MPG on my way to work bumped back to the usual 52-53mpg @ 70mph, 102-108°F outside temp and A/C set at 78°F. Tire pressures really *do* make a big difference.