How com my fuel milage fell from 50mpg to 42mpg

Could be a few things, this must be a hybrid, 1st make sure all the tires are inflated properly, the tire pressure will be stamped on the side of the tire, also if it a hybrid it has regentave breaking system which means the more you use the brakes the more power is returned to the battery system, also the higher the alcohol content in the fuel you buy the lower the fuel mileage, what i recommend is to fill the car with the best fuel you can get NO ALCOHOL so you can get a base line on the fuel mileage then try other brands and grades to see which ones give you the best mileage.