Premium is best. I have run these engine on the dyno, premium always makes the most power. Its your engine, you going to save 10-20 cents per gallon, but risk damaging the engine from detonation?. For example, if your car holds 13 gallons(maybe more), at 15 cents more gallon, thats $1.95 per tank. Please at the very least use mid grade 89 octane if you are that cheap, but in that case driving a Focus would be better. The Mustang is a performance car, treat it like one.
If your stock then 87. If your stock and use 91 or 93, your performance can go down. I've seen test's too. Also the higher the octane number the harder to burn (some think opposite for some reason) and the unburnt fuel can damage your catalytic converters if you have them. and your wasting money. If your car has a tune for higher octane, then definitely use it. If your up the compression or timing, like cars with a distributor, then you might want to up the octane. Some cars can advance the timing once the EEC detects a higher octane, though for mustangs, that's the 05 GT's and up (seen tests too). Before that, the EEC can't determine if you have 87 or 93 octane.
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1995 Mustang GT-Heavily Modded-Sold 2003 Ford Lightning-Sold 1997 Saleen-5.4 conversion-Sold 2005 Mustang GT-Torch Red-Sold 2003 Mustang Cobra-Oxford White 2003 4Runner V8 Sport Edition 4X4
What does 99-04 gt fuel 
