I don’t want to look a gift horse in the mouth, but my gas mileage has gone UP 5 mpg since it has warmed up.
During the winter I was getting between 20-22 mpg on a 90 mile a day drive.
Now that summer is here, I’m getting 25 mpg on the same drive.

I’ve not done anything to the car other than oil changes. But I did one in the winter, and it did not impact the MPG.

To add to the confusion. Since it has warmed up, I run with the t-tops out.
I would have thought this would have caused the MPG to go down, not up.

Any ideas what would cause the mpg to go up in the summer. I don’t want a drop of 5 MPG a gallon once it gets cold again.

 

There are several factors involved...

EFI gasoline engines use an air temp sensor to help calculate a/f mix. Warmer air holds less fuel, colder air holds more (thus one reason turbocharged motors benefit from intercoolers). So the warmer the intake air, the less fuel the car will use (to a certain point).

Also depending on location, gas stations will use "winter grade" fuel in the winter ( HowStuffWorks "Summer-grade versus Winter-grade Fuel" ).

Cold temps can also change / lower your tire pressure, which will adversely affect mpg.