Octane switch at pump

Hi. Many gas stations use the same hose/pump for different levels of octane. If you fill up after someone who pumped low octane gas, and you choose high octane, what volume of the gas you pump is low octane? This is more of a motorcycle question since I only put 3-4 gallons into the tank. Been wondering what amount of high octane stuff I’m really getting.

Well, do the math. Figure that the diameter of the hose is maybe 3/4", and the length maybe 10’ (120"), so do the formula for a 10’ long cylinder with a .75" diameter:

Pi * r^2 * height = Volume of a cylinder


3.140.140625120=52.9875 cubic inches

And Google tells me that 1 cubic inches = 0.00432900431 US gallons

So 52.9875 * 0.00432900431 = 0.22938311588 gallons of fuel in a 10’ long hose that is .75" in diameter.

Also, that assumes that the entire hose is full of fuel, which I doubt it is.

Since the shut off is right at the nozzle, the hose is probably pretty much full of gas. Nearly a quart seems high, but the math appears correct.

The real question is, how much will this quart lower the octane of 3 or 4 gallons of premium. My guess is, not very much. Wanna try the math on that one?

Find something else to worry about.

What makes you think you are getting 93 octane fuel just because you pushed the button and paid for it? Can YOU test gasoline octane? Do you think ANYBODY ever does?

I won’t say that this doesn’t happen ever, but come on! What do you think’s going to happen when someone fills up their old hi-performance sports car with fake 93 octane and it starts knocking like crazy? Even if they don’t go to the BBB or the state attorney’s office, they’re still gonna want a new tank of real gas, and that’ll wipe out the couple of cents per gallon the crooked gas station made on the people in cars with knock sesnsors.

It’s just not a scam that makes much sense.

Many, many “Premium Only” cars run FINE in 87 octane “regular” fuel. The difference between 87 and 92 octane is just not that much, not enough to make an engine “knock like crazy”. In a modern car, the knock sensors can completely suppress audible spark knock. (if required)

With todays .30 cent a gallon price spread (what a rip-off) the incentives are huge and the chance of getting caught are slim…

Some high performance cars may indeed run better on premium fuel, but few will destroy themselves if the owner mistakenly or intensionally uses regular fuel.

Unscrupulous station operators can easily take advantage of this…

Maybe Consumer Reports should take a sampling of “Premium Fuel” nationwide and test its octane level. That might make for interesting reading…

All but 4 states have at least yearly octane inspections at the pump. These inspections are not scheduled in advance. The inspector shows up when he wants and tells the clerk he will be running octane verifacation on pump #X. Some states have a bi-yearly inspection program. Failure in Washington can cause a station to be closed for weeks. Compare the cost of closing for a week to what can be made cheating a consumer out of .30 a gallon. It is really not worth it.

Few states have the time, money or equipment to test octane. That is changing however as a new method of testing called NIR is becoming available. Routine verification of octane may become more common…

New Mexico, a state that claimed to test octane but did not actually do it, has switched to this method. Ohio and 4 other states have no provisions to test for octane.

They don’t have any choice. The law to inspect octane ratings at the pump are on the books in all but 4 states. Octane testing happens everyday. You can even contact your state government for a list of pumps tested in calender year 06 and the results of those tests.

If I had an issue like that with a motorcycle which only has a 3 gallon tank, I’d fill up with regular and pour in a half can of octane boost. Rocketman

Wait behind the Mercedes.


If you wait behind mine you’ll get diesel. (-;

I’m with you. My bike really needs 93 octane to run well. It is immediately obvious if I run 87 octane. My tank is about 2.5 gallons so for me, the residual in the hose is a higher percentage of the total volume of fuel. I don’t like it but there’s not much choice, I have to buy premium and settle for the amount I can get on a “fillup”. Adding octane boosters isn’t really an option either. I fill every single day when I ride and there’s no place to store additives on the bike. The gas stations I frequent do not sell them either. They’re low budget stations out in the boonies…

You’re lucky you didn’t ride dirt bikes with me in the 70s & 80s . . . 2 strokes. You couldn’t carry a 5 gallon can of pre-mixed gas along . . so what did we do boys? Dumped in the oil at the gas station (while looking for the cops, no license on the dirt bike), fill-up the tank, shake it back and forth . . and yiiiiiiinnnnnnnnnnggggggggggg!!! Or dump in the oil before you got to the gas station, then fill-up (I know, an oil rich mixture will gum things up, so get there before you use all the gas in the lines. I echo what I said, if it’s so much of a problem, dump in the octane booster when you start your bike, then fill-up whenever. Or fill up and leave room for a pint of octane booster and dump it in when you get home.

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This is not a trivial question for small tanks. I found this when looking up the issue for my chainsaw.

Mine needs higher octane and you don’t mix much at a time unless you are a busy wood cutter. I was going to get 2 litres of fresh high octane gas. As for the volume of the last selected grade you get, it is what’s in the visible hose plus the hidden piping back to the pump’s selector valve.

I’ll be dumping 1 litre into my car to flush the hose of low octane stuff then put my 2 litres into a gas can leaving a bonus hose full of high octane for the next lucky user.

There are a few gas stations left that have a separate hose for each grade, far and few between though.

There’s actually a station that is near where I used to live that sells E0 premium and regular along with E10 regular and plus. they only use one filler hose, so the last time the PA department of weights and measurements came by, they required the use of additional signage warning that the E0 wouldn’t be E0 until about 1 1/2-2 gallons (until the bowl inside the pump got drained plus residual in the hose) had been pumped. If it were me, I’d do what @Robb_Ellwood said, put 2 gallons in my car, then put the rest into the gas can.

My point exactly about the E10. I don’t use any E10 in my small engines. So when I fill a portable can how much E10 is in my container? The E10 destroys equipment in small engines and the engines. If you can’t find a dedicated filler you are going to get an E10 mix in your equipment.