weather and ground. just like the other guys mentioned it. tire wet, dry, low tread wear, low tire pressure(bad alignment), tire size are all factors of tire that effects ur performance and chirp.
wet or dry, up/downhill, sticky, cement, rocky etc...
but if you really have power loss, you should feel power loss on every gear, every rpm.
remember weight distribution also, and remember that ur car is gt, so there is no lift to worry about.