I have an easier time believing that your car was the one that ran 15s. I just don't see how a pretty much stock mustang ran 13s. Heck, low 14s are factory freaks if its equipped with an automatic. Most automatics i see run are 14.5+ and 14.5 auto is with a good driver. Either that or i'm almost sure that the sensors were acting up, i've heard of it happening quite often