Gas mileage gone bad.

[ProjectM96]

I have had the car for almost 2 years now. Daily Driver.

I drive it the same way everyday. Half spirited(shift at 4000-5000 rpm range), half slow. Mixed city and highway-mostly city driving.

In the past week, I noticed my gas mileage had dropped dramatically during my daily commute to/from work. The gas gauge needle goes down so much after such a small distance. I did not change anything or add any weight to the car. No gas leaks from what I can tell. Engine idle is good, no CEL light, drives and runs smoothly. Oil level is good. Air filter is new. Tire pressure is correct.

Basically after 18 miles of driving, I had used up 2 gallons of gas. I have been averaging 9mpg this past week when I usually get 18mpg average on my daily commutes.

Are there any other reasons for the decreased gas mileage? It has been very hot this past week. 90+Fahreheit. Not sure if heat could affect gas mileage that much.