Found it. Since the gas goes through the same hose as the other blends, if you're buying gas you need to get at least 4 gallons so the previous gas flushes out what was left in the hose from the previous sale, and fills enough of the container that the blend is primarily what you selected.

If you buy E15 and fill your tanks, then the next guy buys E10 and only gets 1 gallon, they might end up with a higher ethanol mix than intended, and if they use that on a small motor, like a chainsaw, it could ruin the seals.

These signs also exist in the Netherlands, and the reason is because pumps are calibrated to a certain accuracy (say, 1%, for convenience).

But in the real world, you can get a lot of variation from temperature, to how long it's been since a pump was used, to how full the underground tank is. They all made a difference, mostly at the start and end of pumping.

So you get a law in percentages, and you get a real world deviation in volume. Obviously, if your pump is short 50ml on 1 liter, you're off by 5% and breaking the law. But of you slap a sticker on, telling everyone they "must" get 5 liters, you're off 50ml on 5L, a perfect 1% deviation and entirely within the limit.

But 8 gallon is BIG volume, damn.

Does this pump also dispense marked fuels through the same hose?

In my province of residence gas stations near farming communities often sell “marked fuel” (fuel with an added red dye in it) that are taxed less, and which are intended for farming machinery, road work equipment, boats, and other non-highway use only. If you’re caught with red-dyed fuel being used for any other purpose you can be charged with an offence, and levied fines or other penalties.

If you dispense a small amount of regular gasoline after another purchaser had bought marked gasoline, the dye in the fuel remaining in the lines likely isn’t diluted enough to tell the difference — and you could (hypothetically) then be charged with possessing marked fuel without the proper paperwork.

(Anywhere I’ve ever seen marked fuels sold usually has a separate hose for the marked fuel to be dispensed from to prevent this from happening — but I don’t know your gas station or where you live, so maybe they rely on dilution rather than separation to differentiate?)

I got curious so I duckduckgo'd that shit.

Link

Buying E10 fuel (a mixture that contains 10 percent ethanol) from a hose that also supplies E15 fuel (a mixture that contains 15 percent ethanol) must buy at least four gallons to protect customers following behind. Ethanol is hard on engines and less efficient than regular gasoline. E15 can even cause engine failure in smaller or older engines. So if you’re using a blender pump to buy E10 that sells both E15 and E10, the residual amount of E15 left in the hose from the previous customer could cause significant damage to those smaller and older engines—unless you purchase at least 4 gallons.

Source

There's actually a really good explanation for this.

This is a mixing pump. It mixes ethanol into the fuel. Because the mixing happens before the part of the pump that measures how much is being dispensed, you need at least a few gallons to fully flush things out and get somewhere close to what you're actually buying.

Nobody is going to come arrest you if you buy 2 gallons of gas. But the gas you get me not be the mix you wanted.

Fucking 15% ethanol now? So they basically just raised the price of gasoline 5%, and reduced the fuel efficiency of everyone's vehicle, without so much as a peep. Where is this?

Which is it, up to 15% or up to 10% ethanol?

It either violates federal law or it doesn't. May is doing a lot of heavy lifting here. Not to mention there are signs for both up to 10% and up to 15%...

If the purpose of the warning is to make sure people get the blend they want then just say that.

Fuck you if you own a motorcycle, I guess.

My RV200's tank only holds 1.7 gallons, and my CH50 will only take one. I guess I'm meant to just dispense the remaining three gallons onto the ground, then...?

Ethanol is wildy bad for the environment and is raising the cost of food.

I read somewhere awhile back that it takes something like 3 gallons of gasoline (through running farm equipment and transportation) to make 4 gallons of ethanol. That coupled with ethanol being less efficient that gasoline and causes more wear and rear on vehicles probably means that if we ditched it altogether we'd probably the same carbon emissions wise.

Also, for every acre of corn raised to go into ethanol, that's one acre not going into feed corn or other food crops so we're effectively raising the cost of food via limiting supply and competition.

The only people that benefit are farmers that recieve substantial subsidies to grow it and government personnel who administer said subsidies and elected officials that campaign on taking money from you the tax payer and funneling it into these programs.

The cost to all of us is diffused, probably no more than a few tens of dollars taken from us via taxes so nobody is gonna go stand up to these people and demand that we end this subsidy. The benefit to them is very focused and large so they have every interest to keep the cash flowing their way. Every interest to take money from all classes of people but most damaging to the poorest of people since those dollars mean more to them than richer peoples.

This wildest part is that you who is reading this right now is probably outraged that this program is in place but the even crazier part is that you can substitute this with practically any and all industries and they are all doing the exact same thing but for the things that you agree with, are perfectly fine with keeping those subsidies flowing. After all, it's only the other people that are greedy. The only real solution is to completely end all federal subsidies but I'm sure you'll disagree and say that XYZ is necessary because it's your special interest.

how do they enforce that? seems undoable

I think it has to do with how it’s dispensed. If the person before you bought regular fuel, the hose still might have residual fuel from the other kind, meaning you could end up with a different ethanol level than expected. That’s a fire hazard. 4 gallons is probably overkill but better safe than sorry.

Edit: basically, if you accidentally get half a gallon of 10% ethanol fuel and half a gallon of 15% ethanol fuel, you actually have 12.5%.