185/65/R14 on 2001 Prius

I have 185/65R14 Nokian i3 on 2001 Prius. This width tire is accomodated by the wheel wells with no interference. I even suppose that there is room enough for Z-cables, but have not actually tried them.

Based on circumfrence, your odometer/speedometer reading will be about 2% slow (if it was accurate to begin with), and the fuel economy 2% low. You typed '14' instead of '65' for the profile I presume?

Your speedometer is not adjustable, or if it is, Toyota has not revealed the secret.

The above is a separate matter from the fuel economy 'hit' from wider tires, which will probably be about the same magnitude. Hard to make a more certain prediction on mpg, as it depends on that tire's construction and (markedly) on how you inflate them. The first 1000 miles or so on any new tire are a bit worse, until the sharp edges are 'rounded off' a bit. If you have previously been keeping mpg logs, you'll know the truth after about 2000 miles.

Actually tires have the least rolling resistance when they are just about out of tread. But as they also offer the least traction, it is not in your overall safety interest to run them that way. Except during fuel economy competitions perhaps?