Well, i am not sure how long they have been offering the two different packages. But I am pretty sure the '05+ the differences are mainly on the standard interior packages. Including the stereo, leather vs. cloth, dash cluster(???). Beyond that, I don't really know
I only really know the difference for the 2003 Model year since this is what I own. You own one as well, so maybe it will help you to.
2003 Gt Premium
Standard features
Bullitt 17" wheels
Mach 460 Stereo
ABS/Traction Control
Leather seats
2003 GT deluxe
standard features
Painted 5-spoke 17" wheels
Single CD AM/FM radio
no ABS/TC
CLoth seats
The deluxe is more of a stripped down premium. You CAN order the options on the deluxe and equip it like a GT premium, but the only option you can't get on a Deluxe is the Bullitt wheels.
All GT's from 1999 have T/C and ABS. The interiors and wheels are the only differences. The deluxe has standard cloth, no Mach 460, and the '99 style 17'' wheels. They started two levels in 2001 at the earliest. The premiums came with fleather, Mach 460, and Bullitt style wheels. For 03/04 prem you could get optional 'bright alloys' which were the 99-01 Cobra wheels. You could upgrade your deluxe with options like the Mach 460, or leather. Most were built with plain stuff only. I can't think of any deluxe's with upgraded goods. It was cheaper just to buy a premium if you wanted that stuff. The Mach 1000 was only for premiums. The reason behind the two levels is because it is faster and cheaper to build cars as packages, not individual optioned cars. Most auto makers realize that the majority of people who upgrade things want it all. Those that like plain cars don't want any of the stuff. Just be glad Ford quit putting 16'' wheels on base GT's. I have all the brochures from 1994 to 2004.
Not true. Like i said, i only know for the 2003 Model year since i have the brochures, but the 2003 deluxe did NOT have ABS and T/C standard. You had to option it out. Also for 2004 it was an option for both the deluxe and premium. It was not standard.
I know this because i live in New England and needed to make sure my stang had the ABS/TC. I looked at a few brand new 2004's and found they did not come with it. It was an $800 option and not many came so equipped. I ended up looking at used 2003's and found that deluxes also did not always have it.
The window sticker for my 2003GT premium lists the ABS/TC as standard along with the leather, mach 460 and bullitts.
All my '99 thru '04 books list T/C & ABS std on GT's. My '03 GT Deluxe had T/C & ABS std. I am looking at the window sticker now. My '02 GT Premium has T/C ABS as std. All my friends with GT's ranging from '99 to '04 have T/C & ABS. Some Deluxes, some Premiums. For '99, '00 and early '01 there was no Deluxe and Premium. Just the GT with whatever options it was ordered or built with. V-6 Deluxes didn't have T/C, only the V-6 Premium. You may be looking at the wrong engine package. I even have the dealer only books for these cars. I swiped them from the dealership I used to work at. They have sales data, who buys the cars, all available options and combo's, etc.