they dont add horsepower. But they do make a stock car faster. With a 275/40r17 and a stock T5 o/d of .68 a 3.08 is gonna top out at 228 mph. 5100rpm which is where our engines stop making power in stock form is 187. Niether one of the cars will go that fast. however adding a 3.55 gear 5100 rpm will come around at 162 mph, i believe a stock mustang with an exhaust and a 3.55 can pull that high. Top speed will be 198 mph.