actually i believe the standard is 18% (which is a mix between the manual and automatic). Its just how much they underrate the engine. 210hp with an 18% loss is 172 at the wheels. so if its 191 at the wheels, that equates to around 225 at the flywheel. Underated about 15hp.
the gt's ive seen put down 276 at the wheels stock, thats bout 325, so the gts are about 25hp underrated.
My matco tools guy use to work at a dyno plant for ford. He says the way they use to come up with the hp #'s was dyno numurous engines, take the one with the lowest hp and take about 10% off of that. He says each engine can vary 10% in power (10% of 300 is 30hp at the flywheel)