Changing mine has corrected the optomistic speedo to a more realistic speed as confirmed by GPSEmm said:Hi poppy1
I haven't fitted any diff tyre sizes due to the rolling circumference changes between the 2 tyre sizes you mentioned 215/40/17 are 1850mm, and 205/45/17 are 1875mm.
The standard tyres should be 205/40/17 1815mm.
I'm no tyre expert but i'm sure this will effect the speedo or gear ratio's, but if you are more interested in protecting you rims (mine are scuffed to......) then go for it
The Cir Fiq are based on one brand and it may vary