Jim in Ohio

I have used regular 87 Octane in my 2006 RX400h for two years. for the last 12 months my MPG average is 27.6 (I do not reset so that I can see winter/summer averages over time, but the dealer reset it about a year ago). Using 87 octane I usually do about 8% better in the summer than in the winter for mileage. I tried higher octane for several tanks across the same weekly driving habits and did not get any improvement worth the extra $.20 per gallon. The manual says minimum 87 octane or higher so I am comfortable that there is no extra engine wear due to knock. I have good driving habits. My '99 RX300 averaged 21.0 MPG. I am enjoying the gas savings.

I've been reading this thread and thought I would put some info up for everyone to consider. The AKI, (Anit-Knock index) or "octane rating" of a fuel ia an average of 2 different methods of computing the fuel's ability to resist engine knock. Regular unleaded as I am sure you are aware has an anti-knock index of 87 and Premium is in the 91-93 octane range. the higher the number, the more anti-knock protection the fuel offers.

Engine knock happens when the fuel in the cylinder ignites by itself before the spark plug ignites it. The term for that is "pre-ignition". It can sound like marbles rattling in a coffee can , and it generally gets worse the harder you press on the gas pedal. Often times, due to the isolation and more advanced engineering, you probably can't or won't hear this occuring. But rest assured, it is.

Knocking can indeed damage your engine because what is actually happening is the fuel and air mixture is igniting and exploding, pushing down on the piston before it reaches the top of it's stroke and is free to move downward again. Thus making the engine work against itself. This places large amounts of stress on certian engine components such as pistons, connecting rods and bearings. In some cases knocking can burn holes in pistons and cause head gasket failures.

The reason for using a higher octane also pertains to the burn rate of the fuel. The higher the octane, the slower it will burn. this is because the fuel is less volatile, and requires more heat to ignite, thus offering more control during the combustion stroke. Your engine was engineered with this in mind.

If the owners manual says use a higher octane fuel, then listen to what the good book says. You will almost allways see that requirement in higher compression higher performance engines.

On another note, the "Premium" gasolines does not in itself make it any cleaner than regular gas to burn. Alot of oil companies would like you to believe that thier premium fuels have some special formula used only in thier premium fuels to help clean injectors, remove deposits, restore lost power, and even help with the dishes and clean the windows. the truth is the same amount is put in all thier octane fuels for standard commercial uses.

Which brings me to my real point here. Sorry if the above is long winded. But what truly amazes me is that the real issue that effects us is the use of alcohol, ( Ethanol) in fuels. It's a much more complex question than which octane fuel to use. Alcohol/gasoline blends may make a car look better in an emissions test, but actually there is alot of debate on the pros and cons of using them, let alone mandating the use of them! As most emissions tests don't take into account that you car will burn more fuel in an ethanol blended fuel. On a per gallon basis, your car will emit fewer pollutants with Ethanol blended gasolines, but on a per mile basis you will burn more fuel, thus offsetting the emissions gain. Which is the more important factor as it just raised the cost of fuel with no benefit. For example, if you get 30 MPG's with straight gasoline, and 28 MPG's with a blended gasoline, for every 1000 miles you drive, you would use just over 2 additional gallons of gasoline then if you used straight gasoline. So, even if the Ethanol blend burned 5% cleaner, (they claim 3-5%), your still actually polluting more with the blended fuel as you burned an additional 7% to acheive that. Not to mention that Ethenol can degrade rubber components such as gaskets and hoses. Most mfgr's have addressed this though. But older cars may not be so well adjusted.

I realize also we are talking very small %'s here. But, 2% here, 1% there, and your right back to spending the few pennies you think you may be saving along the way, and most people never add the cost of repair to the cost of fuel over the years. Did that head gasket blow for no reason? Did that catalytic converter just need to be replaced because it was just old and worn out? Why is that car not able to put out as much hp anymore? Hmmm, could it all be related to saving 2 dollars a tank on fuel?

I've been reading this thread and thought I would put some info up for everyone to consider. The AKI, (Anit-Knock index) or "octane rating" of a fuel ia an average of 2 different methods of computing the fuel's ability to resist engine knock. Regular unleaded as I am sure you are aware has an anti-knock index of 87 and Premium is in the 91-93 octane range. the higher the number, the more anti-knock protection the fuel offers.

Engine knock happens when the fuel in the cylinder ignites by itself before the spark plug ignites it. The term for that is "pre-ignition". It can sound like marbles rattling in a coffee can , and it generally gets worse the harder you press on the gas pedal. Often times, due to the isolation and more advanced engineering, you probably can't or won't hear this occuring. But rest assured, it is.

Knocking can indeed damage your engine because what is actually happening is the fuel and air mixture is igniting and exploding, pushing down on the piston before it reaches the top of it's stroke and is free to move downward again. Thus making the engine work against itself. This places large amounts of stress on certian engine components such as pistons, connecting rods and bearings. In some cases knocking can burn holes in pistons and cause head gasket failures.

The reason for using a higher octane also pertains to the burn rate of the fuel. The higher the octane, the slower it will burn. this is because the fuel is less volatile, and requires more heat to ignite, thus offering more control during the combustion stroke. Your engine was engineered with this in mind.

If the owners manual says use a higher octane fuel, then listen to what the good book says. You will almost allways see that requirement in higher compression higher performance engines.

On another note, the "Premium" gasolines does not in itself make it any cleaner than regular gas to burn. Alot of oil companies would like you to believe that thier premium fuels have some special formula used only in thier premium fuels to help clean injectors, remove deposits, restore lost power, and even help with the dishes and clean the windows. the truth is the same amount is put in all thier octane fuels for standard commercial uses.

Which brings me to my real point here. Sorry if the above is long winded. But what truly amazes me is that the real issue that effects us is the use of alcohol, ( Ethanol) in fuels. It's a much more complex question than which octane fuel to use. Alcohol/gasoline blends may make a car look better in an emissions test, but actually there is alot of debate on the pros and cons of using them, let alone mandating the use of them! As most emissions tests don't take into account that you car will burn more fuel in an ethanol blended fuel. On a per gallon basis, your car will emit fewer pollutants with Ethanol blended gasolines, but on a per mile basis you will burn more fuel, thus offsetting the emissions gain. Which is the more important factor as it just raised the cost of fuel with no benefit. For example, if you get 30 MPG's with straight gasoline, and 28 MPG's with a blended gasoline, for every 1000 miles you drive, you would use just over 2 additional gallons of gasoline then if you used straight gasoline. So, even if the Ethanol blend burned 5% cleaner, (they claim 3-5%), your still actually polluting more with the blended fuel as you burned an additional 7% to acheive that. Not to mention that Ethenol can degrade rubber components such as gaskets and hoses. Most mfgr's have addressed this though. But older cars may not be so well adjusted.

I realize also we are talking very small %'s here. But, 2% here, 1% there, and your right back to spending the few pennies you think you may be saving along the way, and most people never add the cost of repair to the cost of fuel over the years. Did that head gasket blow for no reason? Did that catalytic converter just need to be replaced because it was just old and worn out? Why is that car not able to put out as much hp anymore? Hmmm, could it all be related to saving 2 dollars a tank on fuel?