Text Size:   A+ A- A   •   Text Only
Site Image
Motor fuel frequently asked questions
General motor fuel questions
What standards does Oregon have for gasoline and diesel fuels?
Oregon adopts motor fuel standards developed by the American Society for Testing and Materials (ASTM). These are nationally recognized standards and are used by regulatory agencies and industry alike.

What other indications may be important to me regarding my vehicle and the gasoline I use?
Cost per mile of gasoline used. You may find that purchasing a lesser expensive gasoline may actually cost you more to use than a more expensive one. If you purchase a gasoline that costs $1.899 per gallon and get 26 miles per gallon, then you may be actually be paying more than if you purchased a gasoline that costs $1.929 per gallon but you get 30 miles per gallon.

To calculate the cost per mile, take the cost of your gasoline fill up and divide it by the number of miles driven since the last fill up. That will give you the cost of gasoline per mile driven. With this information, you can determine if the lesser expensive gasoline is really less expensive to use. For example:
  • You purchase gasoline at $1.899 per gallon. You have driven 250 miles since your last fill up and it takes 9.6 gallons to fill your tank for a total of $18.23. You calculate that you got 26 miles per gallon.
  • To determine the gasoline cost per mile, take the total purchase price of $18.23 and divide it by the 250 miles driven, it cost about $0.0729 per mile for that gasoline. ($18.23/250 miles = $0.0729/mile or 7.29 cents per mile)
  • Now, lets say you purchased gasoline at $1.929 per gallon. You drove 250 miles since you last fill up and it takes 8.3 gallons to fill your tank for a total of $16.01. You calculate that you got 30 miles per gallon.
  • Again, take the total purchase price of $16.01 and divide it by the 250 miles driven, it cost about $0.064 per mile for that gasoline. ($16.01/250 miles = $0.064/mile or 6.4 cents per mile)
  • That is a cost per mile difference between the two gasolines of only 0.89 cents per mile. But multiply that difference times an example of 15,000 miles driven in one year, that is about $133.00 savings by using the more "expensive" $1.929/gallon gasoline vs. the $1.899/gallon gasoline.

BUT, that is based upon your particular vehicle, operated by your particular driving, and that you actually achieved more miles per gallon using the higher price per gallon gasoline. You must carefully calculate the cost per mile for your own circumstances and determine if this is right for you.

Motor fuel complaint questions
I think I bought bad gasoline or diesel fuel, what should I do?
  • Immediately call us at 503-986-4670 or submit a complaint online, so that we can obtain a sample of the fuel as soon as possible from the same dispenser that you bought your fuel from. Thousands of gallons of fuel are dispensed through modern fueling sites today and multiple refilling of the storage tanks occur frequently. It is very difficult to get the very same fuel that you bought.
  • Ask for motor fuel quality complaint form from our office, complete and mail it back to us as soon as possible. Include a photocopy of the receipt, if you have one.

Will you tell me what the test results were?
Only if you have completed a motor fuel quality complaint form in its entirety and returned it to our MSD office.

What is my recourse if my engine is damaged by poor quality gasoline or diesel?
You will need to seek advise from your own personal legal counsel.

What do I do if I feel I have been shorted at a gas station?
We commonly receive complaints from consumers because they believe that a gas station wrongfully charged them for more fuel than they actually received. While this may be the case, there is often a misunderstanding of the amount of fuel that can be dispensed into their vehicle. Please visit our Understanding fuel tank capacity page for more information regarding possible reasons for variations in fuel tank capacity. You may contact the Measurement Standards Division to speak to someone and have a complaint form mailed to you or submit a motor fuel complaint form online.

Motor fuel testing questions
Can you test the gasoline or diesel from my fuel tank?
No. Oregon Administrative Rules requires that an official sample be obtained from the source of the fuel such as directly from the dispenser nozzle.

Can I have a gasoline or diesel fuel sample tested myself?
Yes. You will need to contact a motor fuel laboratory, collect your sample according to their instructions, and get the sample to their laboratory all at your own expense and risk.

Storage tank questions
Is water permitted in the station's fuel storage tanks?
Yes - within specific limits. In aviation gasoline, alcohol-blended gasolines, and biodiesel, the maximum amount of water allowed in the fuel storage tanks is 1/4 inch. In non-alcohol blended gasoline and diesel fuel, the maximum amount of water allowed in the fuel storage tanks is 1 inches.

How is the amount of water in the fuel storage tanks determined?
A "water finding" paste is applied to the end of a long probe or "stick" and it is lowered to the bottom of the storage tank. This paste is very sensitive to the presence of water and changes to a bright contrasting color where it has come into contact with any water. The area where the color has changed is measured and recorded.

What happens when the permissible amount of water in the storage tanks is exceeded?
The fuel dispensers that are affected by that particular storage tank with the excessive water are immediately removed from service and the person in charge of the business is notified. Once the water is removed from the fuel storage tanks, the dispensers are placed back into service.

How does the water get into the gasoline and diesel fuel storage tanks?
There are several ways water can enter into the fuel storage tanks. Most storage tanks are under ground and the fill pipe is at ground level, closed with a tight fitting cap, and is covered by a protective lid. Water can seep in around the gasket of the cap or through the spill valve. Water may also be inadvertently brought in with a delivery of fuel to the tank. Or there may be a loose fitting around the top of the storage tanks that allows high ground water to enter the tank. In any case, it is the responsibility of the business to assure that water does not enter the fuel storage tank and if it does, find and correct the cause of the problem.

How does water get into my car or truck's fuel storage tank?
Water can get into your vehicle's fuel tank by a number of different ways including from fuel that you have purchased, natural condensation on the inside of your fuel tank, a loose fill cap on your tank, someone's mischief, and others.

How do I get water out of my car or truck's fuel tank?
This may depend on how much water is actually in your fuel tank. Very small amounts could probably be managed with "fuel-dryer" additives, which combine with the water in your fuel and simply run it through the engine. If the water problem is serious, you should have a competent mechanic resolve the problem. This may include removal of the fuel, fuel storage tank from the vehicle, cleaning it, replacing the fuel filters, re-installing the tank, and filling with fresh, clean fuel.

Gasoline octane questions
What is gasoline octane?
Octane is a measure of a gasoline's antiknock performance, in other words, its ability to resist "knocking" which is a metallic rattling or pinging sound that results from uncontrolled combustion in the engine's cylinders.

Why is gasoline octane important?
It may prevent engine damage. If a gasoline is used with too low of an octane rating than is required by an engine, then engine knock may result. Heavy and prolonged knocking or pinging may cause power loss and may cause damage to the engine.

How is the gasoline octane rating determined?
Gasoline is subjected to two ASTM testing methods to establish its octane rating. The research method (R), runs the gasoline through a special test engine without a load. The motor method (M), runs the gasoline through a special test engine under load. The octane rating that you see posted on the gasoline dispensers is an average of the two test methods. Thus the R + M/2 number that you will see on the yellow and black octane sticker. For example, 87, 89, or 92.

How do I know what octane gasoline to use for my vehicle?
Check your vehicle's owner's manual. Regular (87 octane) gasoline is recommended for most cars. However, some cars with high compression engines such as sports cars, older cars, and certain luxury cars, may need mid-grade (89 octane) or premium (91 or 92) octane gasoline.

How will I know if I am using the right octane gasoline?
Listen to your car's engine. If it does not knock or ping when you use the recommended octane, then you are using the correct grade of gasoline. If your car's engine runs well and does not knock or ping when using a lower octane gasoline, then there will be no advantage in switching to a higher octane.

If your engine does knock, it does not necessarily mean that there is something wrong with the gasoline. It may be a problem with engine's ignition timing or exhaust gas re-circulation (EGR).

Should I ever switch to a higher octane gasoline?
Sometimes engines will knock or ping, even when using the recommended grade of gasoline. When this occurs, try using the next higher octane gasoline for a few fill-ups. Typically, the knocking will stop. However, if the knocking or pinging continues, then your vehicle may need a tune-up or other repair.

Will my vehicle run better on high octane gasoline?
No, not necessarily. Using a higher octane than recommended by the vehicle manufacturer will provide no additional benefit to the engine. You may find that your gas mileage or power increases. In which case, you may get the same result from an engine tune-up.
Some newer vehicles have electronic knock sensors that may cause you to notice a slight improvement in performance when using a higher octane gasoline.

What else influences my car's octane requirements?
  • Temperature: Hotter air and engine coolant increases your engine's octane requirements
  • Altitude: Higher altitudes decrease your engine's octane requirements
  • Humidity: Drier air increases your engine's octane requirements
  • Engine spark timing: If your engine's spark timing is increased, the octane requirement increases
  • Driving method: Rapid acceleration and heavy loading increase your octane requirement.

Fuel grade questions
Is all regular or premium gasoline equal?
The octane rating for gasoline labeled as "regular" or "premium" is not consistent across the country. To ensure you know what rating you are buying, look at the octane sticker on the dispenser instead of relying on the name or the grade.

How much will it cost me to use a higher grade of gasoline?
On average, it costs about 5 cents per octane point between regular 87 octane gasoline and mid-grade 89 octane. It costs about 3 cents per octane point between 89 octane and premium 92 octane gasoline. If you are consistently purchasing a higher octane gasoline than you need, this money will add up quickly.

Miles per gallon questions
What are "miles per gallon" and how do I calculate it?
The term "miles per gallon" or MPG is a measurement of how many miles your vehicle traveled for each gallon of gasoline used. To determine miles per gallon, fill the fuel tank of your car to the top and write down the miles shown on the odometer. Drive your car as you normally would until you have run the fuel tank down and you need to fill up again. Fill the fuel tank to the top and write down the miles on the odometer. Subtract the previous miles from the current mile reading and that will give you the miles driven from the last fill up. Then divide the miles driven by the number of gallons you just purchased and that resulting number equals the miles per gallon that you got since your last fill up. For example, if you drove 250 miles since your last fill up and it took 9.6 gallons to top off the tank, then 250 miles/9.6 gallons = 26.04 miles per gallon.

It is important to keep track of your miles per gallon over time and calculate an average since your tank may be filled fuller at one stop vs. another and driving conditions vary. An average miles per gallon calculation will give you a better idea overall on how your vehicle is running.

What does the miles per gallon tell me?
The miles per gallon that your vehicle is getting will give you an indication of how efficiently the vehicle is running. If you see a dramatic drop in your average miles per gallon, then there may be a problem with your vehicle and it needs competent service.

More information on fuel
For more information see our Motor fuel information center.