ok, so it's not a toyota, but still.
My Mrs Kia Picanto(1100cc Automatic, 57kw) used to get ~400km/30L which slowly got worse and worse to the point that it got to ~330km/30L. Then, after taking it for a trip to chch(4hrs at 100km/h), it is better than ever at ~440km/30L
Any ideas on what would cause this?