The original car's owner replaced the stock rims with 19" ones (see attached pic).

The tires came with the car are 255/45/19. I replaced them with Michelins same size.

Now I need to replace them again but the same size isn't available. The available sizes are 245/45/19 and 255/40/19 (which is very thin for our imperfect streets which are 100 times worse than NYC's).

Will 245/45/19 fit?
What determines the size of the tires?

In other words, how much flexibility do I have in terms of tires size?

 

245/45/19 will be just fine. it's a bit narrower and the profile is a little lower, but it's not going to affect anything. overall diameter isabout 1.2% difference, that's nothing at all

 

Thanks rominl.

What should I expect to gain and lose by reducing the width of the tires?

And how do I know the perfect size of tires for my rims?

 

narrower tires mean you have less traction, technically. but unless you are really pushing the limit, i think it's fine

there is no "perfect" size, it's always a range. honestly i don't think it will be a big deal