My Corolla computer is about 2MPG optimistic.
Unusually cold weather, Winter gasoline, and Winter tires have all brought my mileage (as calculated by ME) down from 34-36MPG to 31-33MPG. I've seen this before, it will start going up soon.
Cruise Control only improves fuel economy for the heavy footed and in bureaucrat's minds. If traffic allows, coasting over the top of hills, picking up speed in the downhills, and holding a speed up hill that doesn't force a downshift, you can easily beat the CC for fuel economy.