Bad plugs, leaking injectors, bad oxygen sensors, broken vacuum lines, dirty filters, clogged injectors, heavy foot, faulty fan clutch, bad wires, fly on a MAF sensor (seen it), sticking throttle body, bad math, low tire pressure, old fluids, frozen brake calipers, superchargers, poorly made air intakes, rock and roll, Honey Boo-Boo, or ****ty quality gas.