particularly on motoerway runs, wheel size, tyre size, tyre inflation and a clean egr/fuel system can all make a big difference. Under pressure tyres can increase frictio AND reduce the wheel diameter/circumference.

These can all add up to detriment of your MPG.

 

I run 195/45R16 tyres on my 2011 fabia. they give a nice compromise of rolling speed, handling and ride comfort.