I just ran into the same situation. Apparently the gas station pump accounts for fuel density (changes in volume due to temperature) while the on-board computer only accounts for fuel flow.
that's why i always buy gas at night. when it's cooler and dense. plus using the fuel pump on the "low flow" notch on the nozzle gets you more fuel than "high flow" notch; less air to be sucked in and registered as fuel... there is a write up on it. I'll try to find it later...
By cleaner I meant more efficient and motor safe, which it is. But given the amount of fuel additives in most high octane deliveries, it IS a clean burning fuel. As far the internal motor parts are concerned.
And as most know by the time the fuel hits the combustion chamber taking in to account density and tempurture....91 becomes a couple points lower in it's actual octane rating as does 87.
So the higher the better.
It might seem odd that fuels with higher octane ratings explode less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation as opposed to the ability of the fuel to burn (combustion).
Thus it is possible to formulate high octane fuels that carry less energy per liter than lower octane fuels. This is certainly true of ethanol blend fuels (gasohol), however fuels with no ethanol and indeed no oxygen are also possible.
Alcohol fuels such as methanol and ethanol, are partially oxidized fuels and need to be run at much richer mixtures than gasoline. As a consequence, the total volume of fuel burned per cycle counterbalances the lower energy per unit volume, and the net energy released per cycle is higher. If gasoline is run at its preferred maximum power air/fuel mixture of 12.5:1, it will release approximately 20 MJ (about 19,000 BTU) of energy, where ethanol run at its preferred maximum power mixture of 6.5:1 will liberate approximately 25.7 MJ (24,400 BTU), and methanol at a 4.5:1 AFR liberates about 29.1 MJ (27,650 BTU).[citation needed] To account for these differences, a measure called the fuel's specific energy is sometimes used
The best advice is to always run the lowest octane recommended for your car by the manufacturer; in your case, that's premium fuel. In the case of a Shelby GT one has to wonder why you'd spend 35 large on a vehicle then try to save pocket change by using lower octane. How much will you save? Someone can check my math, but in my area premium is $0.20/gallon more than regular. If you get 18 MPG overall (not unreasonable) and drive 12,000 miles a year you'll save a whole $133 each year by burning regular. Not worth it to me--you make up your own mind.
91/93 octane doesn't burn as cleanly as 87 octane.
93 is more combustible and last longer. NO SUBSTITUTE. If it wasnt better Ford wouldnt tell you to do it
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