Look in your owners maunal it should tell you but I think it would be prem or mid. The reason you need the higher grade is because of the way your car in programed.
I think the recommended Octane is 87. The octane rating is just the fuels resistance to detonation (knocking in the engine when the fuel ignites before the cylinder is at top dead center). You only need higher octane when you run higher compression, forced induction, or nitrous.
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1979 Mustang Ghia-302 HO @ 300hp, holley 600 carb, edlebrock intake, under drive pulleys, Bullet proof C4, 8.8 inch rear end with 3.55, Flowmasters Delta 40s American Thunder 2, pioneer head unit with sony speakers.
2004 Mustang Mach 1-281 DOHC @ 305HP, Shaker, 5 speed, 8.8 rear end with 3.55, Mach 460, Sat radio, Flowmaster Delta 50s, Clear Bright Fog lights, alum items for interior dress up, and an eject button.
Fobra is right... you're just wasting your money with anything higher than 87 if you're running a stock set up. Unless you're pinging for some strange reason under load then 87 is all you need.
A good quality 87 octane is more than sufficient and just as clean as any higher octane fuel.
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66 Coupe Tahoe Turq - 200, Pertronix & Flamethrower, 3 Row Rad, Cliff. 6/2 96 GT Vert Laser Red - FMS pulleys, Bullet ProM, BBK CAI, Prof. Prod. Elbow and 70mm TB, MAC Off-Road H, Borla side exhaust, Tokicos, B&M Ripper, 18x10 & 18x9 DD Bulletts 06 GT Tungsten, UIP, Red Leather, Stick, 18" blades, Steeda CAI, SCT XCal 2 w/ MC Racing 91 tune, MACs, BMR SP010 springs, BMR Adj Panhard, CDC ducktail & quarter louvers, Rear louver, painted black stripes
For your stock GT, 87 octane will be fine. When you start modding with a tuner or FI then premium is recommended. Note that the tuner can be adjust to except 87 but you won't get much performance out of it as you would from 91 or 93 depending on your state's premium octane rating.
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2003 Mustang GT Fobra, Sonic Blue 4R70W 262rwhp/280rwtq - Mods in profile Vortech coming soon.....
Daily Driver 2001 Mustang V6, Oxford White 4R70W
Mods in profile
What octane rating do you recommend for a stock 2000 Mustang GT 
