My ethernet 10/100 physical (KS8721) chip tracks to the RJ45 integrated magnetics connector should have had a differential impedance of 100 ohms according to the manufactures spec, ( 5 thou tracks and 5 thou gap). However the board was laid with 10 thou tracks 5 thou gap giving
70 ohms differential impedance (ground plane 5 thou below). The tracks are about 25mm long and the board seems to work ok but I am about to do a few changes and wondered if its worth changing, 5 thou tracks are very thin and I have noted that the manufacturers development board also uses 10 thou tracks.