Found some info about wheel diameter for certain tyres, all recommended by Lexus:
195/65 R15 = 615mm
205/55 R16 = 596mm
215/45 R17 = 625mm
This makes that we have some differences with mileage and speed shown by the car - a bit higher speed and distance shown when using 205/55 R16.
Also MPG will be a bit better.
Still this does not explain why 15" do better with fuel consumption (as in the 1st post).
I am also using Michelin Cross Climate, 2nd year, and very pleased with.