The only practical way of tracking fuel economy is driving duplicate courses for many tankfuls with each fuel. Complications like changes in seasonal air temperature, fuel type (Winter versus non-Winter gasoline), traffic, etc., all complicate getting a good picture of actual fuel economy.
For example, my commuter vehicle gets used almost identically for my weekly commute, and hardly at all on weekends, yet my MPG calculated at fill-up time varies by almost 2MPG. However on Winter gas it averages almost 2 MPG less than on non-Winter formula gas, but it takes several fill-ups to find the new average.
Premium gas only increases fuel economy in situations where the knock sensor invokes timing retardation while using lower octane fuel. If that condition is not occurring (and it's hard to tell if it is or isn't), then changing to a higher octane gasoline won't help.
If you tow, race, or accelerate up steep hills often, you may see a small increase in fuel economy.
Another complication in tracking fuel economy, is that it takes a little time for the engine system to "learn" that it has higher octane fuel. How much time depends on the system, and how often knock is occurring.