i run 91 octane gas

Depends what the mods are...search this forum there are a lot of threads on this.

You should be fine with 87 as long as you don't have anything like a ship, tuner or timing adjuster that will advance your timing. Also if you have a blower or N2O you should be running 91.

You should be okay with 87, but if you add any type of forced induction, you will need 93 octane.

The SEARCH button is your friend.

Run as low an octane as you can without pinging or detonation. Anything more is a plain waste of money.

Why spend the extra cash....only your pocket book knows for sure.

You are wasting your money if you do not have a tune for 91 octane.

Thats BS if I would tell you to use 87 or 89 or 93 octane.....

The only one who can find out is you:
Get 87 octane gas and drive your car on a warm, dry day, AC on.
Cruise for about 5 minutes (on interstate what so ever) and then put the hammer all down.
If you hear engine ping, which actually sounds more like someone ratteling with a bounch of keys below the hood, betwen 4000 and 5000 RPM @ WOT you know that you would be better off with higher octane gas.

Check that out and you'll know!

Here's what i have Heard so far n gas and Mustangs.............95-99 stangs use 89 octane
(mid grade) from the factory.......I you put 93 octane in any ofthese cars stock they will die and not stay started.........2000-2005 mustangs are equipted with a better puter that will allow the use of 87-93 octane........I have to use 93 or my 98 will really pig and sputter.....but It's sc'd and chiped........

I would stick with factory gas ratings untill you get more mods>
JMOP,
TK