check out www.tyresave.co.uk/tyresize
to see how wheel diameters are affected by changing tyre sizes, it seems to say that standard 16'' mcs
wheels 195/55/16 are 5mm bigger in diameter than the S spokes on 205/45/17, only less than 1% but may be enough to affect speedo and mpg trip comp calculations. i find mine 10% out on the 17''inchers.
Smaller wheel travels less miles per gallon at the same engine revs than a slightly larger one.
But also being wider has more contact with road so economy also affected by this.
Any body got better knowledge of this/