I posted a little table to compare the tires but it is being moderated....
It boils done to about "1": RPM will drop 1%, speedo will drop 1 mph at 60 to 59, and Odo will read 1.67% off....all well within the tolerance limits of the instruments. In fact those who have noticed that their speedo is a bit optimistic would now read on their speedo a number closer to what the radar gun or gps claims. As for fuel mileage will a 1% RPM drop (about 30 RPM at speed) make any difference? I kinda doubt it. As of rolling resistance I imagine there are lots of variables there but I doubt the very minor size difference will account for much.
This is all comparing a 195 to a 205, so that is a 1cm difference in width and 5mm difference in sidewall height.
I suspect if you strictly used your instruments to compute mileage all these numbers are a) very small and b) would balance out. Of course real mileage has to be computed by real distance (which is hard to know without a well calibrated odo (this change could make it more or less accurate depending on where it started)) divided by real fuel consumed which for most of us requires some averaging over many fill ups using the pump number.