i'm running 42 x 16 and will probably gear down to a 40 x 16 because of my knees. Normally it works quite well if i am going at full speed. However, if i am going into a headwide or just riding at a leisurly pace, my knees kill me. I have pain aboe and to the inside. doc gave me a bunch of exercises. You need to stretch out your hammies really well. put your foot on the top of a chair and reach for your toe (do NOT bend over and stretch your back muscles). then stand straight up and hold your foot with your hand behind your back and push against it with your foot but do not bend over.