I like to change my pulsar 180 classic tyres 18 to 17 alloy, hw much it will effect on my bike....?

If you are planning to go with the original alloy ones from bajaj, then it might result in a mileage drop of around 3 kmpl, which should not matter. Other ones not too sure. If your P180 is old enough there is no point in investing about 6-8 k for the Alloys. Moreover, you may have to change the tyre as well. It might burn a big hole in ur pocket. Think again before buying..