Let's assume the dealer gives me my GT filled with 87 fuel but I want to use 91 grade. Can I drive the car until it's almost out of gas (like 1/8 tank) then just fill it up with 91? Or do I have to do something special?

 

run it empty and fill it with 91/93

 

unless it's tuned, you can run a mixture. It can be 10 miles to empty or 100.
It runs on 87 and it runs on 91+.

 

Will running 87 in my GT reduce fuel economy?

 

no, octane just means a more efficent burn and better for the engine. when applicable.

There's a lot of info in this thread- tho it's regarding the 2010, it's principles stay the same.
https://themustangsource.com/showthread.php?t=478748

 

Never a good idea to actually "run it empty." Tough on the fuel pump.

 

And I am not a fan of pushing cars to a gas station.

 

No offense, but I have a friend who works in a SVT dealership. Him and me always laugh at the people with too much money who guy and buy these whipple mustang GT's from the dealership, who have no idea about detonation or octane levels in Gas.


Even tho they tell them to only use Husky or Shell, and only 91+ they still blow up their engines half the time lol.

 

ill take your word on that.

 

nothing like taking something literal for a talking point. you are smart.

 

As long as your car isn't tuned for a specific grade of gas, you can run '87 through '93 without any issues. My car has a '93 octane tune and I need to stay with that grade.

 

wish we had 93 here in az.

 

We can get 100 - 104 at some stations, but you need to drive to get it.

 

You just run the tank down til your next fill up, and put the desired gas in. ECU will adjust.

 

Didn't believe me huh?

Mix and match, run it near empty then refill, whatever... the ECU and the Adaptive Spark Ignition System will adjust the timing to avoid knock. That's why its called adaptive.

I have not noticed any reduction in mileage.

Have you downloaded the Owners Manual pdf yet?

 

Hm. Slight tangent here.... I don't know about anyone else, but I can tell the difference between 93 and 87. 87 (and 89) knocks badly, 93 doesn't. It bothers me a lot when I hear spark knock.

I want to say I see an up to 2 mpg improvement vs the 87 too, but maybe I'm being delusioned.

/maybe it's just me...
//Oh, yeah, and +1 on the run it out and start using what you want.

 

Quote:

Originally Posted by houtex

Hm. Slight tangent here.... I don't know about anyone else, but I can tell the difference between 93 and 87. 87 (and 89) knocks badly, 93 doesn't. It bothers me a lot when I hear spark knock.

I want to say I see an up to 2 mpg improvement vs the 87 too, but maybe I'm being delusioned.

I never had knocking w/'97, but There is a drivability difference when I used better gas...

 

Just tell the dealer you want to drive it down and fill it up yourself. My dealer just handed me $60 dollars and said have fun. I brought back $20 and a receipt. He said i could have at least stopped and got us lunch. Small Town Dealers are a little different LOL. At a larger dealer you will likely have to take your salesman with you to sign a ticket but he would have had to have taken it for fuel anyway.

That said its not going to hurt it (Other than a 2.5% loss of power) to run 87 Octane. In truth you would do well to tell the difference on the street. Loss of fuel mileage might occur at part throttle depending on how Ford has the car tuned. If they take full advantage of the extra octane at part throttle to run more timing with premium then it will get worse with regular. But then again its not going to be a huge difference.

Based on the way the new Copperhead PCM is supposed to try to advance the timing a few times and if it experiences knock each time it goes to a setting a given number of degrees below that. So it should run the least timing on 87 but if you say refill at half tank with 93 and the resulting mix was 90 it would adjust for 90 fuel not default to 87 setting (Oversimplification: Due to the diminishing returns as you add more and more chemicals to raise octane a 50/50 mix would likely be closer to 91 octane).

Bottom line dont stress over it! You are not going to permanently retune your car by 10 bhp by running a tank of 87 through it.