As a long time member, and many questions answered by this group, I felt an obligation to pass on what I have recently experienced.
Four or five years ago I installed Goodyear Viva tires, from Walmart, and had I felt exceptional service. It was time for a new set, so I purchased again the Goodyear, but with model name Viva "Authority". This was advertise by Goodyear that you would gain X amount of fuel savings over the life of the tire.
I have always been satisfied with the MPG which was on the road or highway driving from 24 up to 28.
We had an occasion to take two round trips from Bullhead City to Qaurtzite, AZ. The first trip of 232 miles the display showed 34.7 MPG Ave.
The second, I filled the tank "top off" to double check. Again the display showed 35.1 and used 6.64 gallons.
The only change in the car was the tires.
From the start I felt that there was less effort to maintain speed, but it could have been imagination.
Anyway, I hope I have not bored you with this.