tire size for 01+ civics

I have 17's with 205/40/17 tires and I was just wondering how this will effect my speedometer odometer and gas mileage. I know this tire size is about a half inch to an inch smaller in diameter than stock so I'm worried. Or does it not really make a difference?

http://www.miata.net/garage/tirecalc.html type in your stock tire size and the new size and find out how off it will be.