2) Ethanol has a higher octane rating, but a lower energy content than gasoline. That means that it pings less, but it will give you a lower gas mileage. E85 yields about a 30% decrease in gas mileage over pure gasoline. Generally most people cannot detect the gas mileage difference between pure and E10. This is evidenced by those who believe that they get more gas mileage with the E10. The laws of physics generally contradict this result.

So for example I might see

$3.13 for 87 octane
$3.23 for 89 octane
$3.33 for 93 octane

I hadn’t even considered the possibility someone still had to deal with that.

Anybody know if rear fuel fillers are no longer allowed to be made in the US or is it a voluntary design change?

Also, this alleged “mid-grade” costs 10 cents,or 3% less than regular, but in both my machines yields 10% less mileage. Not a winning proposition in 3 seasons. I buy it anyway in winter and skip adding HEET–no gas line freeze yet.

BTW, back in the days of the old 4-way Conoco pumps, a pump jockey (this was before self-service was legal) got confused and put 11 gallons the bottom grade, not the top one, into the 20 gallon tank of a car that called for premium fuel.

When nothing bad happened as a result, I started buying the 3rd grade, rather than top grade, and eventually got up the guts to drop to the 2nd grade on a near-empty tank, which did make the car knock.

From then on, I tried to buy gas at 1/2 full and to alternate grades 2 and 3 to make de facto 2 1/2.

The 10% rule applies across all cars and trucks I have owned in the ten years I’ve been measuring. Granted, when you’re getting 17mpg in the V8 Tundra I drive now, 10% is a little over a 1.5 mpg difference. That seems dangerously close to the margin of error, and I do my best to control for the variety of driving conditions I see, but with the large sample size I think this can be considered a fact.

I did not realize that mid-grade is simply a mix of regular and premium. I am now wondering if there is a cheaper mix of the two that I could use to eek out another mpg or so… I will have to look into it.

Mid grade is made from blending in the pump. They really only hold low and high grade. Sunoco used to have selectable blends on their pump all the way to 100+ Sunoco 260. Nice pink stuff, smelled like lighter fluid. crack cocaine for piston heads.

You make a good point. I’m a pretty thrifty guy, but I never thought to look at whether stations are charging higher than the difference between 87 and 91 for 89 octane. I’ll pay more attention to that in future.

Since you asked, my wife likes using pay-at-the-pump for gas but I don’t. (I also don’t like the self-serve checkouts at Home Depot.) I suppose if I get stuck switching fuel grades mid-fill to get my 89 octane, pay-at-pump would be the only way it wouldn’t be a PITA though.

Even without stopping mid-fill, refuelling my cars is not something I enjoy, and not just because of the cost. The location of the fuel filler neck, behind the rear licence plate, is one of my only pet peeves with the 1966 Chrysler. It’s way down low, you have to wrestle with the spring-loaded licence plate, and there’s nowhere convenient to put the gas cap while filling. If you’re not listening for the change in “gurgle” when it’s almost full and partly release the filler nozzle, fuel will shoot out and spill on the ground.

I’ll suggest reading this to actually see the impact commodity prices have on food prices. http://www.ers.usda.gov/AmberWaves/February08/Features/CornPrices.htm

alanp: Your math is good but your theory is off. A compressed gas gains heat which in turn raises the pressure. So a 10:1 compression engine will have a pressure of about 370 psi at TDC prior to ignition (assuming isentropic compression). At 10,000 feet, 255 psi; equivalent to an engine with a 7.7:1 compression ratio at sea level.

I don’t know about you but when I’m pumping I use “pay at the pump” with a credit card and it doesn’t cost me more than 30 seconds to switch grades during a fill up.

Besides if you watch for it you might find the (high grade + low grade)/2 cheaper than the mid grade on the same pump.

I suppose you could argue the station would price it differently if the mid grade wasn’t there but I see stations in my town that have 2 low grades and 1 premium on the same pump and their premium prices aren’t abnormal compared to the other stations.

Think of it this way, if you have a 10:1 compression engine, and the air pressure at sea level is about 14.7 psi, then in the engine it gets compressed to 147 psi. If you’re at 10,000 feet where air pressure is about 10.1 psi the cylinder pressure at full throttle is only 101 psi. Effectively you’re driving with the same power, engine pressures and ping possibilities as if you were at sea level but the engine compression was only 6.8:1 – a very low compression engine that would not need much in the way of high octane.

Now IF you drive a vehicle with a turbo or supercharger that is able to keep the intake air charge as high as if the car was at sea level, you’ll need the same octane at altitude as you do at sea level. It’ll just be harder to find the high test gas.

I’ve wondered why the pumps had lower octane here. I’ll have to try the 85 and see if I notice any difference. Thanks all here for the inputs…

“Ethanol’s energy content was not found to be a direct predictor of fuel economy.”

This is of purely academic interest, giving no indication of any benefit over straight gasoline.

“E20 and E30 ethanol blends outperformed unleaded gasoline in fuel economy tests for certain autos.”

This is in fact CONTRADICTED by the evidence in the study, for all vehicles except the flex-fuel Impala. The Toyota and Ford each may have gained 1% in fuel economy using E30 over straight gasoline, but 1% seems to be well within the (undeclared) margin of error for the tests so it is not a scientifically significant result. Besides, that is only the result for the highway drive cycle. In the city cycle they each lost a statistically significant 6% and 8% efficiency, respectively. The non-flex Impala burned 8% and 5% more fuel in highway and city drive cycles when run on what the report calls the vehicle’s “optimal” ethanol blend. The flex fuel Impala did impress by gaining 15% and 14% fuel efficiency over straight gasoline by using E20, but it suffered a 24% loss on the E85 highway cycle. Of course, by now the credibility of the entire report must be questioned.

“Standard, non-flex-fuel vehicles operated well on ethanol blends beyond 10 percent”

That claim is simply pulled out of thin air. The study only states that they stopped tests using higher concentrations of ethanol in the fuel when the check engine light would come on under full engine load. That is a long, long way from saying that the vehicles operated well on any fuel (they ran Chevys fer cryin’ out loud!) and says nothing about the effects on the longevity of the engine.

With increased altitude, pre-ignition tendency is reduced. In the old days, we would advance our ignition when we hit the Rockies. That’s why you can use lower octane gas.

b: My car needs midgrade. A lot of cars may be specced for 87, but due to wear, sensors dying, or just cruddy manufacturing, they need 89 to not knock.

As for mid-grade gas, I’m with the previous poster who suggested that the pump should be able to mix that on the spot with standard and premium gas (if it doesn’t already). Since it isn’t subsidized by the government (like Ethanol) I guess the market will decide if it’s viable or not.

My Father used to work for Sun Oil back in the 80’s & 90’s and they used to have 5 grades of gas (at least in the Mid-Atlantic) 86, 87, 90, 92, & 94 octane. He used to tell me that the pumps weren’t accurate enough to blend the 87 so essentially you got 86 and the station made a little extra profit.

Please, PLEASE do not get rid of mid-grade gas!!!!!! At higher altitudes, regular gas is 85 octane and we need to pay mid-grade prices just to get 87! Mid-grade is all I buy!
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drooartz :
March 22nd, 2008 at 12:10 pm

Unless you live in Utah, where the grades are 85 – 87 – 91. I do run midgrade here, but that’s just to get the minimum 87. I can’t afford any power loss in my daily driver Hyundai Accent — now don’t laugh, it’s paid for.

I’ve never quite figured out why they run lower grades at our stations here.

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Actually, with the altidude in Colorado and Utah, 85 octane is equivilent to 87 octane at lower altitudes. I read something about years ago in the paper where they cleared that up. You can Google the info as I am not the expert. Yes, I used to use 87 octane when I first moved to the area, but seeing that 85 octane made no difference in performance, I’ve stuck with 85 ever since. Give it a try. Especially with gas stations in Denver metro starting to post only their prices for 85 on the street sign, which now conveniently allows them to gouge on 87.

I could well be off in my assumptions about the energy yield of ethanol from grain. In the statistic you cite, what are the inputs that are measured to come up with that figure? I would assume that at a minimum they would include fuel used for cultivation, harvest and transport, as well as fertilizer use. What about water use and demands made on the land and topsoil? In other words, were all required inputs for production truly and accurately measured? My inner skeptic still wonders if some of the less tangible and long-term costs were truly considered.

I’ll cut this off at this point as I am again wary of thread-jacking, but I’d suggest that the subject of the true costs of ethanol production as currently practiced in the U.S. might be a worthy subject for further discussion here on TTAC.

As for mid-grade fuel, the few times I’ve used it to avoid preignition and knocking problems were followed by a bit of judicious tuning which usually resulted in my being able to go back to 87. Mid-grade’s a nice option to have when you need it, though.