Bigger tire than most recommended? 8,5inch 265/35?

I need to put the proper tires on my wheels. I have 8,5j now with 245/40/18 tires as i thouht this will work best, but i read that people use 265 and no traction issues ... also with 265 sidewalls look more flat and with 245 more strech ... can you help here from race point of view what i loose/gain if i put bigger tire? Car is street but will track it this year on some events ...

Welcome to the forum Anatoli!

To get the maximum performance on track for a given set of tyres, it is best to go with the rim width recommendation from the tyre manufacturer. As a general rule, the tyre sidewalls should not be stretched in either direction, they should be quite vertical. In saying that, different brand tyres always look a little different from each other.

Try to stay in the middle of the recommended width. Here is an example from Michelin's catalogue where they give you the recommended rim width for each of their tyres as an example. Remember you muse use the recommendations of the specific tyre brand you are using though!