Originally Posted by M3V8Driver
Those two sizes can't have the exact same size sidewall. A 265/35 would have a sidewall about .5cm (5mm) taller than a 305/30, so if you're going by numbers on a website, they're either rounded off or inaccrute. But I suppose they're close enough to pose your question. Given the same brand and model tire in both sizes, there would be very little noticable difference. If anything, the 265/35 might feel more forgiving over bumps, but I think you would only notice it if you had a very firm aftermarket suspension. Assuming the sizes in question are what you're planning on running front/rear, Tte difference in sidewall and overall circumference wouldn't even make a huge difference in RPM (rotations per mile). If anything, you want the rear tire to be taller since they tend to wear out faster. That would give then fewer RPMs. That's what BMW did with the stock fitments. The front 245/40/18s are subject to more RPMs than the rear 265/40/18s, thus sort of equalizing the wear timeframes (unless you do a lot of burnouts
There's also another component here that is not being taken into consideration. While you are comparing overall height there's been no talk about the width and how it affects clearance from the fender and the rear suspension. At 305/30, you might be near stock in terms of overall diameter but you are much wider than stock and depending on the offset of the wheel you could be rubbing the rear suspension components. I don't think I've seen anyone running 305/30 yet and this could be the reason why.