nglitz's rule is definitely true.

And I'll offer what I know to help explain why.

Their are a couple of properties of gasoline I don't see mentioned in this forum thus far.

1) The reasons why you would use the lowest octane rating your vehicle was designed for (besides price)

and...

2) The difference in quality between formulations of low octane and high octane gases


The higher the octane rating is on a specific fuel, the more resistant it is to detonation. Everyone knows that right? It's because the hydrocarbon chains in the gasoline are longer, and are harder to break.

What does this mean for mileage? Simply that the higher octane gas you use, the worse your gas mileage will be. Why? Longer chains of hydrocarbons require more energy to break during combustion (providing resistance to pre-ignition by heat), and less energy is released by them during their combustion (as properly initiated by the spark plug).

Now, that's not to say that you should put 87 in a car that wants 91 because you think you'll get better mileage.

By retarding the timing, the ECU will pretty much negate your mileage increase. You'll be into the pedal more, because of the power difference, and use more gas as a result. And you'll negate the perfomance edge designed into your timing maps.

So if you have a Toyota Tercel, by all means, use 87. It's cheaper and your mileage will be better.

But as we all know.. the higher the compression, the higher the tendency for pre-ignition (pinging/detonation). So, can a Mercedes deal with low octane gas?

Any modern one can, without much short term harm. So why use 91/93?

For reason number 2.

There is a difference between grades of gas that is not accounted for by its octane rating. Generally speaking the higher the grade of gas, the more and/or higher quality detergents are added to it. And this means, less deposits on valves, heads, pistons, etc. Chances are good that you will more than pay the difference should your car require a repair brought on by poor quaity fuel (a duel head redux on a 600SL is 72 hours of labor time by the book, no parts!). Carbon deposits in the combustion chamber can have sharp edges that become very hot, when they are hot enough they'll ignite the mixture in the chamber before the spark plug can. Your engine isn't a diesel (at least for the purpose of this arguement), it isn't supposed to run like one!

Don't take my word for it, ask a chemist, and take a sample of all three octane ratings and have a chemical analysis done on them (if you're anal, like I am). Should run you less than $200. Another good thing about analysis is that if you frequently fuel at the same station, the analysis will tell you how dirty the gas is.

Yep, dirt! And you thought you were just buying gas. Your local station makes their money by selling gas, not by making sure that their storage tank is operating room clean. If you find your usual station very dirty, move on! Dirty fuel clogs filters, destroys pumps, hampers performance, and brings on repair costs you don't have to have.

What does this all mean to you? It means, use the gas the manufacturer recommends. It's what you might call cheap insurance.

So why didn't I just say that?

Oh yeah, and if I had a rental fleet, I'd only use higher octane in the really expensive cars (mb, porsche, bmw, jag). They don't rent many of those.

Rental car places ditch their cars at 20K miles typically. That's the best balance of use and depreciation for a lot of cars.

Sorry about the length, just thought I'd throw in my 2 cents.

Justin