As an added point for people (this may have already been posted but I didn't go through all 7 pages of posts).

Double check your performance by tracking your mileage. On our Tahoe, tracking mileage ofer time showed that I got a consistent 10% bump in MPG using midgrade (89) vs regular (87) but no further bump in MPG using premium (93). Note this was over multiple tanks and multiple driving conditions. Since the cost of higher grades is typically $0.10/gal/grade, at $1.00 per gallon this wasbreak even, at $2.00 per gallon I was saving $0.05/gallon by using mid-grade and at $3.00 per gallon I'm saving about $0.07/gallon. The manual states regular is fine.

So, do the experiment over a few tanks. Your car might not benefit, but it did for me. Plus tracking your MPG regularly will also tell you about the health of your engine. If it starts going down, something is wrong.

Don't forget regulation!
 

Some states restrict the sale of diesel autos because of "pollution". Sad, but true.

True, I hadn't thought of the fact that the American companies do have European counterparts that they can borrow from. However, it's not quite as simple as just taking the European engine, sticking it in a chassis, and selling it... the U.S. has different emissions restrictions than the EU, especially for states that have adopted the more stringent California requirements. Thus the engines and exhaust system would require modifications (and hence money poured into design and testing) before they could legally be sold here.

I think diesel's availability is limited not by necessity, but by lack of demand. There is huge availability for truckers, of course. If more consumers needed/wanted diesel, more stations would carry it. AFAIK, diesel is actually cheaper to manufacture than gasoline (though I may be wrong about that). Biodiesel is even cheaper (recycled kitchen grease!) and if the market were right, could lead to even better availability.

Certainly customer choice and negative perception of diesel play a huge part in this. However, as with diesel fuel availability, this is a vicious cycle: consumers don't want diesel vehicles and think they're no good, so manufacturers don't sell diesel and don't see a reason to develop any better diesel options, so consumers have no reason to think diesel is any good, so...

While it is up to the consumer to tell the manufacturer what they already want, so that the manufacturer can make it (if there is sufficient demand), it also works the other way around... it's up to the manufacturer to make a new product and then convince the consumer that he SHOULD want it and DOES want it. Witness, for example, Apple's iPod... the demand for MP3 players was not all that high before the iPod came on the market. It's not that people were clamoring for a better product, it's that most people didn't really think about it much. Then, Apple put out the iPod and said, "Hey, don't you want one of THESE?" People didn't know that they wanted iPods until the iPods were actually made available.

A similar thing happened with hybrids... people didn't want electric cars, and if they wanted good gas mileage, they got compact sedans like the Echo, Civic, or Sentra. There was no significant clamor for hybrids (although demand for increased fuel efficiency was growing, it was not all too significant until very recently). Yet Toyota and Honda made hybrids and convinced people they wanted them... so much so that there is a waiting list for new Priuses. It's no surprise that two Japanese companies were the ones who put the hybrids out and sold them... all the upcoming American hybrids simply license Toyota's technology. Yet American companies *could* do the same thing as Apple, as Toyota, as Honda... they could develop a clean, fuel-efficient, quiet diesel/biodiesel (or hybrid, or whatever!) vehicle and convince people that, even though they didn't know it until now, they actually WANT one of these! But... they don't do that. IMHO, the American manufacturers are no longer forward-thinking and are unwilling to put money into speculative development. Instead, they wait until the Japanese and European manufacturers have spent their money to establish a reliable technology, and they then license it from them once they see that it actually sells. In the meantime, the Japanese and Europeans establish early market dominance. Unless their mindset changes, American manufacturers are doomed to a long, slow failure in this game.

Interesting, that I did not know. Thanks for the link! ^ It is still true that European cars do use smaller, higher-compression engines, however. :)

Sad yes, except that as I mentioned, poorly-maintained diesels will pollute much more than an equally-neglected gasoline engine. The emissions are also much more harmful due to the sulfur content, hence pollution is very much an issue. Once again, it's up to the manufacturers to create engines that can meet stringent pollution controls... and up to fuel companies to make cleaner fuel. And, it's up to states to legislate that manufacturers comply with these regulations.

(A diesel/biodiesel combination fuel can, in a properly-tuned engine, produce fewer emissions than diesel or biodiesel alone, and possibly even lower than gasoline alone, as well. But legislation doesn't demand such engines, manufacturers don't make them, and consumers therefore don't know that they should want them.)

You are correct. Diesel is a "mid barrel product" along with Kerosene which requires much less refining than Gasoline.

Your explanation is not quite right. It is true that on average, European cars require higher octane gasoline. However, as pointed out by another poster, most of the "difference" in octane rating of European and US gasoline is due to the fact that European gas stations report Research Octane Number (RON), whereas US gas stations report the average of RON and Motor Octane Number (MON). Because MON is typically about 10 points lower than RON, this means (typically) a European rating of 95 RON is equivalent to a US rating of 90.

Moreover, a higher octane rating does not mean that the gasoline has been "refined more." The octane number for a gasoline is a function of the ratio of aromatics (basically hydrocarbons that form ring structures) and olefins (unsaturated hydrocarbons, that is with carbon-carbon double bonds) to paraffins (saturated hydrocarbons, all single bonds). Straight-run gasoline typically has a very low octane rating; it typically needs to be blended with distillates with a lot of aromatics etc. You can also increase the octane rating with certain additives such as tetra-ethyl lead (no longer allowed in the US) and MTBE. In the UK, they add benzene and toluene.

Finally, it is not necessarily true that a car will consume "less gas" if it uses a higher octane gasoline or if the compression ratio is higher. Gas mileage is a complex function of many variables, and there is no direct correlation with octane number. Also, a higher octane gasoline does not necessarily result in lower emissions. In fact, it often is the opposite. Adding MTBE or tetra-ethyl lead will increase the octane number but also increase the emissions/pollution.

Sorry, I wasn't clear when I made that statement. I wasn't referring to less gas due to higher octane fuel. What I meant was that an engine with a higher compression ratio and smaller displacement (which often requires higher octane fuel) can achieve similar performance to an engine with larger displacement but smaller compression ratio... the smaller-displacement engine will typically use less gas compared to the larger-displacement engine, with the same performance. The "less gas" is due to the smaller displacement, not the octane rating. This is a generality and isn't always true, obviously (e.g. when you add a turbocharger to the mix), but that's what I meant when I made the original "less gas" statement.

Sorry, I think I was again unclear. I meant that having the smaller engine generally reduces emissions, but you are quite right, there are a number of factors that come into play. My apologies for any confusion, and thanks for correcting me. :)

I'm not entirely sure what you mean by "requires much less refining than gasoline." Diesel is a "mid-barrel" product in the sense that it has a higher average molecular weight and a higher average boiling point than the gasoline fraction. However, that doesn't mean that it takes more effort/energy/time to make gasoline vis-a-vis diesel.

Refining crude oil essentially is separating the components of crude oil into fractions based on boiling point. The lightest fractions boil off at the lowest temperatures (i.e., gases like methane through butane, followed by naphtha). Next is the gasoline fraction (roughly C7 through C11), then the kerosene fraction (roughly C12 through C15), and then diesel, followed by the heavier fuel oils and lubricating oils. Depending on the type of crude oil being refined, about 40% of the crude oil run through the distillation column will be recovered in the gasoline fraction.

Since the heavier components (the so-called "bottoms") aren't worth a lot, the heavier fractions are typically processed further (cracking, hydrocracking) to break apart the bigger carbon chains to smaller carbon chains to make more gasoline and diesel etc. Notably, one would typically not process diesel further to make gasoline. The gasoline fraction may also be further processed or blended to increase the octane number; and similarly the diesel fraction may be further processed to increase the cetane number of the diesel.

As you can see, gasoline does not require "more refining" than diesel.

This is clearly something that people either disagree upon, or are confused about... perhaps it's just an issue of semantics. However, even answers.com says that:
Obviously, they could be wrong... the Internet is replete with plenty of disinformation and misinformation. The point being that either it's easy to be mistaken/confused on this issue, or we are just arguing semantics. :)

Lamborghini Miura SV:
compression ratio = 10.7:1
gas mileage = approx. 13.9 mpg

Honda Civic Si (2002):
compression ratio = 9.8:1
gas mileage = approx. 27.9

I have to think about it a little more, but I think a higher compression ratio (all else being equal) will result in lower gas mileage. Higher power but worse gas mileage generally.

You said, "all else being equal." I, however, did not. What I specified was, "higher compression, smaller displacement, same performance." The Lamborghini has, what, 2.5 times the displacement of the Civic? And the Lamborghini has, what, about 3 times the horsepower? (I'm ballparking here because I'm too lazy to look up the real numbers, but I'm reasonably confident that I'm fairly close to the true numbers.) So, the two are not comparable because the higher-compression engine has a larger displacement and much higher performance. A more correct comparison would be between two cars with similar horsepower.

(EDIT:I looked up the numbers, and I was damn close with my ballpark estimates. The Civic engine is 1.6L and 115hp, the Lambo was 3.9L and 385hp. Unless I looked up the wrong Civic... but then I'd still be reasonably close. :))

I don't know whether for the same displacement, a higher compression ratio will result in lower gas mileage. It is possible that the higher compression would require a richer fuel/air mixture, thus leading to increased consumption and lower mileage. It's also a function of how fast the driver accelerates, of course, so I'm not sure there's an easy answer to that one.

I have had both versions this card for over two years now and have never exceeded any maximums. I do agree the higher non promo cash back is more money in our pockets (when we get it someday) but the disadvantage to that are the hidden terms not enclosed when you apply for this card and the fact that you have to call them to get the $50 check. I like it all just automatically credited to my account. I spent all day the other day calling Chase trying to get information on the Cash Plus Card rebate expiration dates, cash back maximums, etc. Hopefully there is none - I just ordered the card myself to find out more though...

Interest lost and waiting...
After accruing 5,000 points, cardholders can choose between a $50 check or a $50 gift certificate to leading national merchants...I am sure the check takes 2-4 weeks to mail out on top of that.

See, and thats why I like Discover in the gas situation. I requested my reward (I opted for the gift certificates to up my cash back) and had it in the mail in about 4-5 days. Also, I can redeem at only $20 which is a huge bonus especially for us impatient ones. You do have to watch your rewards balance though and actually request to redeem, but that isn't bad. I'd rather have them do it that way than automatically because I might loose the chance to up my awards with the gift certificate option otherwise.

A while back Discover had a promotion where you would get $10 cash back for using the card for ANY purchase at Sam's Club. I spent about $8, and got $10 for doing it! Has anyone heard of them doing this again? (Warning though, I had to call them about 4 times to actually get the reward credited and I had to save the receipt for 6 moths until it credited)

Although both are internal combustion engines, both designs are different in respects to bore/stroke in regards to power generation. There is a fundamental difference between how each achieve their power. Each has their own school of thought. Therefore, it is almost impossible to compare two different manufacturers/engine fuel/power consumption without taking into their account of the design.

Generally you will find that...
Honda= lower displacement, small bore, long stroke, and high rpm's. This equates out to lower torque numbers, but higher hp numbers.

Lambo= Medium displacement with a square bore/stroke and moderate rpm's.

Regardless if you really want to know, Gas is gas in regards to the amounts of energy per unit of gas. However- the higher the octane, the more kenetic energy it is capable of producing rather than heat- the byproduct. Therefore generally the fuel can be compressed at a higher rate, and will not detonate, thus creating more power per cycle.

I hope you can understand this because i'm exhausted.

Although you might occasionally find some bad gas I think it's pretty much all the same. One thing I do is never fill up at a station where they are getting a gasoline delivery. I figure that any contaminates and water are settled to the bottom of the tank and when they are dumping in new gas it all gets stirred up and sent to the dispensing pump and on into your tank. Don't know for a fact this is true, but makes sense and it works for me!