A 225/40/R16 is going to be 7% smaller than stock, which you will notice in higher rpm at the same speeds.
When I was scrubbing in my 215/45/R16 STS tires (About 5% smaller than stock) I had a lot of fun. The car launches noticeably better and pulls harder. It is also a little annoying on the highway because it adds a couple hundred RPM at higher speeds.
I have a curiosity to see what the 225/40 would be like to drive, but for me I definitely would not want to drive them every day. There is little rim protection there and the speedo error, odo error and mileage penalty are more than I would want for everyday driving. I think 45 series is the better choice of these two.
[This message has been edited by kelley nelson (edited August 02, 2000).]