I have 205/60/15 tires on my 98 camry right now, replacing with 225/40/18... I used pepboys overall tire diameter equation (HERE) and the overall diameter with stock wheels and tires comes out to 25.49. The new wheels and tires come out to 25.09. How much will my speedometer be affected?

What really matters isn't diameter, it's rotations per mile. If the rotations per mile are the same, the speedometer will be correct.