there's quite a few factors and in some cases larger tires can improve gas mileage, i usually see it on highway driving, which at 80 miles per day may be a consideration.
1. tire brand and style could impact more than size. you're changing more than one thing at once so that complicated things -the size and tire type "aggressive", both may impact mileage.
2. if you're in a city area and aggressive driving and dancing on the gas pedal then larger/heavier tires have a greater chance of denting the mileage as the engine constantly accelerates that heavier mass and increased diameter.
3. larger diameter tires can do better on highway mileages and worse on in town driving. the tire "goes further" for each revolution. there's a limit and it depends on vehicle, driving, and other tire changes (width and tread type may also impact mileage). i've gotten better highway mileage with larger tires many times, but usually worse in town mileage. and it's not going to work that way if the tires are heavy, sticky, aggressive, wider, etc and again depends on driving.
mountain verses flat land driving makes a difference too. i find the larger tires can change when the car wants to shift and mileage in the mountains more than flatland.
