2021 Ecoboost HPP ... CarMax topped off the tank for delivery,
but used regular grade gas - 87 octane.

I suppose/guess, previous 1/3 tank was premium added by
previous owner.

So overall, I am down on octane.

Is this a concern?

For now, can run normally on this tank, run it
not full boost, and get gas level way down to E,
then fill with premium next time.

Basic net surfing indicates a can of octane booster
would not help out much.

Thank you.

 

I would Run Down to 1/8 Tank Max So Any Crud At Bottom of Tank Won't get Sucked Up! Likely Not Much Being So New But Who Wants to Suck Air! I Always Advise to Fill at 1/4 Tank Normally To Keep Fuel Pump Happy! A Bottle of Techron, Fords Favorite Additive Would Not Hurt Both Now and At Fill up! Waiting For the Fresh Fuel Before Really Hammering it is a Good Plan! Unless Fuel Was Really Rank The ECM Should Compensate Just Fine!

Btmo!

Kc

 

what does the owner's manual say about octane?

 

From what I can find it looks like Ford suggests to use 91 Octane or higher to achieve best performance. Sounds like it adjust and run just fine on 87 octane, but will not achieve the rated 310hp with that fuel.

 

that's what I was wondering ... Mayhem, check your owner's manual to make sure; but most Mustangs since 2010 will self-adjust for octane down to 87; they make more power on higher octane, but have no problem running on lower octane

I am not sure about the "2021 Ecoboost HPP" ; if it is tuned for maximum performance, it might need the higher octane (meaning they defeated its normal ability to adjust for lower octane)