Reloadron. thanks for the links!! if i will be able to find a refleftive sensor like that, i agree it will be the best. but i dont think because, the wheel i will be measuring, dosen't have "Teeth" but rather holes.
in any case, if the led is 5mm , then a hole of 5mm will be good enough i think...
the wheel will be rotating with 4000rpm (max) , and will have 35holes, which means, 0.4ms , between every hole... it is of the out most important, that no hole , will be lost... :/