The Mustang Mach 1 was reborn in 2003 and was only made until 2004. For 2 short years one of the best modern Mustangs was produced. The Mach 1 had 305hp and 320 lbft and was notorious for being under-rated. The word on the street is that the Mach 1 really produced numbers more like 330hp and 355 lbft. When equipped with a 5-speed transmission and a good driver it could hit 0-60 in 4.7 seconds and clear the ¼ mile in 13.2 seconds. Those are some very respectable numbers and those are stock.
The Mach 1 was built with cool shaker hood that allowed the driver to see the engine rumble. Retro font gauges and comfort weave seats really helped complete the best looking interior of that version of the Mustang.
The patient auto shopper can find some gently used Mach 1s with less than 35k miles for under $17,000. The Mach 1 represents some serious bang for the buck.
The base GT was a moderately quick car, but would lose a lot of stop light drags. The Mach 1 was the sleeper of the Mustang stable. There is also a great aftermarket and group of enthusiasts to help make these little gems go faster.
If you want to do a little extra reading there was a great write-up by a Mustang webzine and a Pontiac webzine where they took a 2003 Mach 1 and a 2004 GTO to the drag strip.