Ok, now i have come across this website, where i calculate the difference in tyre size and so on
http://www.1010tires.com/tiresizecalculator

with my tire size of 185x60x15 being replaced by a 205x45x17 it will be Speedometer Difference: 2.192% too slow and Diameter Difference: 2.15%

I am going to fit my polo with cross polo wheels with this tyre. not going to drop the car at this stage.
what do you guys think?