Gas mileage is not a function
Gas mileage is not a function of weeks, or half-tanks and quarter tanks etc. Fill the tank. Reset trip "A" to zero. Drive the car. Fill the tank. Note how many miles you have driven (trip "a") and divide that number by the number of gallons you pumped into the tank. Typically it will look like this: 319/9.8 = 32.5 mpg. This is what my 2016 EX-T gets. So give the car a chance. It will be fine. Bob