I just bought a 2008 F250 Super Duty Diesel and they came with 325/60-20 tires. The truck has 3:73 gear ratio and my mileage is bad around 13 unloaded on highway. Would my mileage increase if I went back to the original tire size 275/65-20?

What effect are the larger tires having on my MPG and speedometer reading? Thanks
Randy