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Gasoline octane determines its ignition characteristics.  Alcohol is added for emission reduction.  Given the same octane and the same alcohol, all gasoline is the same except for price.  Therefore, always buy the least expensive gasoline.  All of the advertisements saying theirs is better are carefully worded to make sure they really say that all gas is the same.  If you do not believe this, then you have not listened to the ads carefully and read the fine print at the bottom of the screen.

In general, you should run with your tank full.  You should store the vehicle with a full tank.  The tank portion not filled with gasoline is filled with air.  Air contains water.  If stored empty, the water will condense and you can rust the tank from the inside.  This really messes up your fuel filter.

Help Us All Get Lower Prices

With gas prices reaching record highs, you can help lower the price.  How?  Stop buying your gas at the high-priced dealerships and then buy the lowest octane that will run in your car.  These high-priced stations only exist because of ignorant drivers thinking that higher prices mean better gas.  If everyone bought at the Arcos and Costcos, etc., then the High-priced stations would lower their prices and we would have real competition.  Every time you buy at a high-priced station, you hurt the concept of competition and thereby make me pay higher prices.  I do not like that.


Petroleum products expand much more than water with temperature change.  Gasoline is stored in nice, cool, underground storage tanks.  The outside air temperature in the summer is much warmer than inside the tanks.  If you top off, the temperature increase will cause the gas to expand into the gas tank expansion chamber.  If this chamber fills, the gas expands into your crankcase.  If this happens, the gas is wasted and your oil is contaminated.  When the engine runs next, this gas will evaporate off and be burned off by your emission control system.  This is really not a good situation.  Did you ever wonder why you really need a PCV for your car engine?  This is why.  I top off the RV because the extra 10 gallons is 80 more miles on a tank – and I drive 50 miles before it has a chance to expand.

Never park the RV immediately after filling with gas.  The pumps say you should not ‘top off’.  From the time the gas clicks itself off until I finish ‘topping off’ is 10 gallons.  Because I do this, it is critical that I drive immediately or the expansion of the gasoline will cause the tank to overflow.  I shop at Costco, Sam’s, and Wal-Mart.  When I finish store shopping, I move over to the pumps and fill up.  Always fill just before you drive. 


Read your manual here.  It probably recommends 87 octane:

Higher octane usually costs more.  The octane rating is a measure of how slowly the gasoline burns.  The slower, the more controlled the ignition and the more likely to push on the engine down-stroke.  This is a waste of power.  Think of pushing your kid on a swing.  You expend the least effort when you push when the swing has stopped at the top of its range.  Pushing as the swing goes back down wastes your effort.  Same with an engine.

In other words, you will get the most power from gasoline that burns fast at the right time.  Your engine computer controls will adjust to the right time if it can.  Higher-octane gas will just confuse the situation and waste gas.  Lower octane gas will burn at the wrong time no matter how your computer adjusts the timing.

I have found some western states pedal 85 octane gas as Regular.  I have seen this in Utah and South Dakota.  There is either something I do not know or this is bad for your vehicle.  When the book says 87 octane minimum, I believe it.

MTBE and Alcohol

MTBE and alcohol dilute the gasoline for emission control.  They do not contribute to power.  Therefore, when you buy gas with 10% alcohol, you are getting only 90% gas.  Ergo, 90% of the mileage per gallon that you would get if you had only gas.  This means that if you get 8 miles per gallon with real gas, you get 7.2 miles per gallon with alcohol-added gas.  Maybe you don’t care but I look for real gas.  In most cases, the alcohol is only added seasonally.  In South Dakota, the state subsidizes the alcohol so that stations sell both -- with the alcohol-gas cheaper.  I buy the real gas.

You hear the government saying how much better or worse your mileage is depends upon your engine. They are correct.  If your engine is operating properly, your mileage is best with real gas.  If you have tuning problems, the alcohol may change your combustion characteristics to improve your situation.  The proper performance solution is to correct your engine.

I do believe that the additives decrease the emissions.  It is a high price to pay since by far the emissions come from older cars with little or no emission controls.  So the alcohol reduces my emissions from .02 to .01 per cent.  That is great.  Now what do we do with the car that is putting out 5.00% emissions?  Eliminating one of these will be the same as using alcohol in 500 Camrys.

Brand Names

Does your RV run better on Shell?  Arco?  Texaco?

If so, you need your head examined.  In any one area, the gas comes from one or two holding tanks and trucked to all the dealers in that area.  Same gas.  The truck to the dealer may be leased or owned by the brand name company and maybe have the company logo on the side.  It may just be a shiny truck.  Most likely.

The brand may add coloring or maybe additional cleaners but this is highly unlikely as anything of this nature is outside the quality control of the refiner.  This causes control problems for the state agencies.

Gas is gas: buy for price not for pretty signs.  Support your local independent to keep the money in your community.


I used to live in Dallas.  There is a brand there called Diamond-Shamrock.  I quickly learned that this gas was contaminated.  It was always contaminated.  I had a motorcycle and found rusty junk in my filter the first time I went to a Diamond station when the bike stalled a few minutes later.  This also happened at a later time at a different Diamond station.

We stopped buying Diamond-Shamrock gas for the car and the bike.

When we left Dallas in a rented truck and the Datsun 510 car.  My wife was afraid of losing me in the truck so she stopped at a Diamond-Shamrock station.  Bad mistake.  The car ran so badly that she had to stop at the first rest area.  I was waiting there for her.  T he Datsun had a translucent gas filter.  The contents were a bright orange.  We drove the truck into town a bought a few new filters.  We were on the third one when we had used up all of the Diamond-Shamrock orange junk.

I have permanently boycotted any Diamond-Shamrock station since then.  Lately I discovered that Diamond-Shamrock owns the local independent that I favor and is now changing the station signs to Diamond-Shamrock.  My boycott was a waste of time.  I doubt if I would buy their gas in Dallas as I hold grudges permanently.

But, to the point.  Back in the days of carburetors and small filters, gas did not have to be so clean and it wasn’t.  Now with everything computer-controlled and fuel-injected and larger fuel filters, gas is much cleaner.  The regulating agencies have improved.  The refiners have improved.  The storage tanks have improved.

With all these improvements, it is unlikely that you will buy contaminated gas.  But I still boycott Diamond-Shamrock.  And Shell but that is because of their credit policies and not the quality of their product.

As an extra precaution, I start looking at pump prices when the tank gets just below ½ full and always fill before it goes below ¼ full.  If I get bad gas, I do not want it to be the only gas in the tank.

When I had an RV with 2 tanks, I never filled both tanks at the same station.  A little paranoia here can save an emergency stop with clogged gas lines or worse.  But then gas really is better:   it has been a long time since I had any trouble with the gas itself.

Fuel Injectors And Gas Filters

Things evolve here.  Rust and dealer tank contaminants used to be a problem.  My motorcycle has this little screen and it would clog with big rust pieces.  My Datsun has a normal size plastic filter and it would turn bright orange and clog.  I hope Diamond has cleaned up its act.

Cars had basically paper filters in a metal or plastic box and for carburetors this was enough.  Fuel injectors are so much more sensitive that everything has changed.  I suspect Diamond would have class-action suits if it still sold rusty gas.  The filters are now complex and very fine screens.  But this also has its price.  The gas companies put detergents in their gasoline to keep injectors and filters clean.

As far as I can tell most American companies use the same chemical detergents in the same quantities.  The ads you see about super-clean are irrelevant.  But there are some companies using a much poorer detergent.  This detergent itself can clog the filter as it leaves a residue.  You cannot know this from octane or brand. You know this because adding an injector cleaner makes a performance difference.

And note that injector cleaners all have the same ingredients.  Buy the cheapest one you can find at Wal-Mart and know that it is the same stuff in the pretty, odd-shaped bottles.  The long neck has  purpose.

And injector cleaner is NOT an octane booster or other gasoline additive.  It makes no claims about anything.  Its only claim is to clean your injectors.  The additives are just money tossed down the drain.

Pemex (Mexico)

In Mexico I think Pemex uses the cheaper detergent.  I need to add injector cleaner on a regular basis.  The USA (AAA) states that Pemex gasoline has a high sulfur content which will clog your emission controls.  Shortly after moving to Mexico my Tercel emissions test light came on.  I attributed the light to old age but the high sulfur may have helped.   If this is true I think that the car companies are getting away with "cheap".  I never like cheap.  With what we pay for emission controls, designing them for one, narrow, gasoline formula is cheap.


You still have to be careful of the gas you do own.  Gas ages poorly.

If your gas tanks sit partly full for a long time in a humid climate, the water in the air can condense in the tank.  This may cause rust and may cause sufficient water to kill the engine.  A gas additive in this case is good.

Also sitting gas will decompose leaving a gunk.  It is unlikely that this will cause your engine much damage as the fuel injectors clean themselves pretty well.  I do add fuel injector cleaner periodically.  A couple of bottles every 6 months since the tank is so big.

The gunk will clog your generator.

When my RV was stolen and sat for a couple months without running, I had a rust-clogged gas filter within a couple thousand miles.


2005 -- August

Vehicle Smog Checks

They could eliminate smog-checks in favor of highway-side sensors with cameras but the hundreds of garages ripping off citizens would go away and the unemployment rate would go up.  They do not catch all offenders now and the highway-side would also not get everyone -- but the tax would go away and only the violators would be impacted negatively.

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Written:  2003          Updated:  June 23, 2008          Back To Top