tire pressure

15 sounds like your running it half flat. The side wall will have the recommended pressure. As for the rear, every car and tire combo is different. There is no exact pressure set in stone. You have to test, test and test. Move your pressure up or down until your tire leaves a even print from side to side. Heavy on the edges then your to low. Heavy in the middle and no edges, to much pressure. Find the middle pressure where your tire leaves a even print. When it's right you will find a nice evenly feathered edge appearance across the tire from edge to edge.