Hi there,
I have just replaced the 185/60/14 wheels on my Golf with some 175/65/15 wheels from a BMW Mini (quite easy to do - for tutorial
click here
I am now looking at 5% increase in rolling radius, and 10mm decrease in width for each tyre. This should count for something in MPG.
Also, the tyres are 3x Michelin Energy (XGreen) and 1x Pirelli P3000 so all LRR. These are much better than my old "Sava" and "Intensa" branded budgets...
I will be on the lookout for an MPG gain on this next tank!