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!

As Tim said, all tyre manufactures will give you a "spec'" rim, which is their advised width for most applications and it'll be what the actual tyre width was measured on.As a general rule, more tyre on the track the better... to a point, because there is also an increase of mass and frictional losses which will hurt performance.

There is a little lee-way with the manufacturers also listing a minimum and maximum recommended rim size. The tyre may be mounted on a narrower rim if the vehicle's use is more oriented on acc'n and braking and the potential for a small loss of lateral stability may be acceptable - or there may be a rim width limit and the additional tyre on the track is worth it, or something else.

Running the tyre/wheel combination with the wider rim will tend to stabilise it - think trapezoid - but it does put more stress on the sidewall and expose it more. It will usually give more stable lateral grip but may break away more readily when the limit is reached. Sometimes one has a spec' tyre size, or limited clearance, and may find the wider rim has an overall advantage.

All things being equal, I'd run the wider tyres and lightest wheels (without giving up strength) I could afford.