You forget I didn’t start this topic I just responded with my experience to another post questioning lower mileage after certain modification upgrades. And I’ve noticed a lower mpg reading for no appearently reason with those same mods, but only after having them for several months. So the lose of at least 3 mpg, in my case makes no sense. I don’t believe there is a connection to the mods. So there has to be another explanation. But it just seems funny that several others from different parts of the country are experiencing the nearly same lose of mpg, at the exact same time frame.
Have you checked your tire pressure? Over/Under inflated tires can cause your MPG to vary. It can also cause them to wear funny. There are too many other factors that could lower your MPG. As noted, Summer-Winter blend of gas, average speed of your drive, highway or city driving, number of passengers or weight in the vehicle, topping off one tank until it runs out on your shoes..... As @edwardaneal noted, a lab would be needed to control all of the factors to determine the true cause of your drop in MPG.