I would think you would get much more information from the lens forum.
what do you reccomend for me then please?
i own a panny FZ150
panny GX1 with 20mm and 45-200mm
i have the 45-200mm for sale, it was not enough for me last week so im having ago at DSLR kit
cant afford anymore than 400mm lens
It's been my experience that for anything other then small court sports, if you're only going to have one camera and one lens, then bring a zoom. In motorsports especially since the "court" is so large. It's an expensive lens I know, but I've never been dissapointed by my 100-400L on full frame. On your camera it would be a 160-640, so think about how often you're going to be trackside when the short end may be too long. With crop cameras the 70-200L would be 112-320 and the 75-300 would be 120-480. Possibly in your case a little more versatile. There are also different versions of both of these lenses at different price points.