In the past I've always noticed an increase in mpg as it got warmer but that is not the case with my current car, a 2007 Nissan Versa with CV transmission. Mileage goes up in the spring and fall but drops in the cold of winter and the heat of the southern California summer. Running without the a/c on is not an option since I have my elderly parents and/or my two little girls in the car all the time. Temperature was 106* and humid here today.
Another factor with the warmer weather, some stations finally get to reduce the "Winter Mix" of up to 10% ethanol.
You would be surprised at the difference in mileage between 5% and 10% Ethanol content. That is one reason that I don't fill in Seattle anymore if I can help it, they have more Eth in their fuel than we do in Olympia. (on average, ask your attendant.)
The counter effect is that it is more fun to drive in the warm weather.
There are several factors at work here, first and foremost, the more fuel efficent your vehicle is the greater relative impact using the A/C will have. A/C takes horsepower, horsepower takes energy to create in the form of combustion or electrical drain. If your car got 10 mpg and you turned on the A/C which takes about 10 hp per hour then on the freeway your mpg would only go down about 1.4 mpg to about 8.6 (using 60 mph / 7 gallons.) If your car gets say 30 mpg, then that same 10 hp for an hour would reduce your mileage by 10 mpg to 20 mpg (using 60 mph / 3 gallons.) So the better your starting mileage the greater the impact of A/C usage on your mileage. For the record, 10 hp was a good figure about 15 years ago, but I think many of the modern systems are more effiecent somewhere along the lines of 3-7 hp depending on the application and amount of volume being cooled, so 10 hp is just an example because it makes the math easy.
The second factor is the ethanol in California gasoline. Before MTBE was outlawed, I could tell you exactly when the first "summer blend" gasoline was used at the stations that I fill at because the summer blend had ethanol in it and my mileage would drop like a rock when the alcohol was added to gasoline. Now since MTBE has been outlawed, there isn't the strong drop in mileage with summer blend because the ethanol is there year round. I've had similar experience to ac7ss this year noting that my mileage has slightly improved during the summer months due to an apparent reduction in the amount of ethanol in pump gas in Southern California. That may be regional though and some parts of CA may just have the same E10 gasoline year round to make it easy for the refinery and gasoline industry.
Good luck and keep your fingers crossed for cooler weather or better gas.
I live in Las Vegas Nevada and when I purchased my car in october I was averaging around 38-40 MPG. Now in the summer where it has consistently averaged around 113 degrees my MPG has taken a drastic decline in fuel economy. I know average around 31 MPG. I do run my A/C constantly due to my kids being in the car.
I'm pretty sure the A/C is the main reason but the heat in general could be a partial culprit.