SGU Listener writes in this question:

I about lost all my friends this week at a dinner party where the topic of acetone in the gas tank came up. Five of my friends, all who drive high-end BMW’s, claimed that when they pour a mixture of 2 ounces of acetone for every 10 gallons of gas, they increase their mileage to approximately 120 miles per full tank of gas. None of them can tell me why this works, but they all “swear” they’ve diligently been logging their miles and see the increase. There was absolutely nothing I could do to change their minds. For every website that showed this was false, ten others claimed it’s truth. They told me if I new so much I wouldn’t be a single guy who drives around in a Town & Country.

Though an ad hominem attack, it was quite effective. So, after much yelling and getting all their wives mad at me, I had to leave the party. Alone and very hurt by the Town & Country comment.

First, Eric, since you live in Las Vegas the obvious solution to your problem is to go to TAM6 and get some new more skeptical friends.

There are actually two interesting questions in your e-mail: the first is whether or not acetone increases gas mileage; the second (and more interesting to me) is if it doesn’t why are your friends so sure that it does?

I too looked around the intertubes and found sites making the claim for acetone and those debunking it. I found the debunking sites to be much more credible. But first let me summarize the claim. Acetone is a common chemical, it is the active ingredient in nail polish remover because it is an excellent solvent. It is highly volatile, meaning that it easily evaporates and turns into gas. The claim made for acetone is that it decreases surface tension and therefore helps the gasoline to vaporize more completely in the combustion chamber and therefore burn more completely. I have seen claims from 10-70% increase in fuel efficiency using acetone – although most say 10-30%, and one site conservatively said 2-10%.

When confronted with such a claim there are two basic skeptical considerations – what is the plausibility, and what is the evidence. On the plausibility side, we have to break down the components of the claim. The claims sound plausible, at least superficially. No one is claiming any magic, or putting forward only vague technobabble. Getting fuel to burn more completely by making the combustion process more efficient makes sense. There is negligible energy in the acetone being added, so any added fuel efficiency must come from more complete combustion. But from there the claims get very implausible.

If adding acetone increases fuel efficiency by even 10% that means that 10% more of the fuel is being burned, which in turn means that without acetone no more than 90% of the fuel is burning, and 10% is being exhausted as unburned hydrocarbons. This is demonstrably not true. Most cars already have a combustion efficiency of > 98%. Therefore there is very little room for improvement, and any claim for more than a 1-2% increase in efficiency by this method is simply not plausible. Further, gasoline and diesel already have a very low surface tension – so there isn’t much room for improvement there.

We can also analyze the probability on the basis of the industry. If significant gains in fuel efficiency were possible by such a simple method I would think that the auto industry would be all over it. There is sufficient competition among various automakers to create an incentive for such an improvement, and increased fuel economy is a big marketing point these days.

What about the evidence? Well, all of the evidence I could find to support the claim for increased fuel efficiency was anecdotal – meaning that people reported they tried it and it worked. Many make note that they carefully recorded their miles per gallon over many tanks of gas, some even trying different amounts of acetone per gallon. The results? Mixed.

The majority of the reports were positive, but some were completely negative. Others shows a minimal effect of 1-2%. Now – let us consider what this means. There is likely to be a huge “file drawer effect” or selection bias in what gets reported, meaning that people who found an increase in fuel efficiency are more likely to report it than those who found no effect. Therefore the distribution of positive and negative results is consistent with the null hypothesis of no effect.

I also found it interesting that many people reported that they were conducting “scientific tests” of the acetone claim, but none of them did. Some reported doing a baseline – running a few tanks without adding anything – but many did not. But I could not find a single report of anyone doing a blinded test – meaning that they recorded their gas mileage without knowing whether or not acetone was added to that tank of gas. Why would this be important? Isn’t gas mileage pretty objective? No.

There are many variables to gas mileage – whether or not the air conditioner is running, highway vs city driving, the weather and road conditions, etc. Also, driving style affects mileage – rapid acceleration and breaking uses more gas than smooth acceleration. A blinded test would control for these variables, but no one, apparently, thought to do that. This reflects, in my opinion, the poor understanding of true scientific methodology among the general public.

There are reports of industry tests of this concept – in laboratory and highly controlled conditions. The result – no effect from acetone on gas mileage. For example, Tom and Ray Magliozzi, who write a car talk blog, say:

TOM: It’s bo-o-o-o-gus, Wil. With four “o’s.” Don’t put acetone in your gas tank.

RAY: It does absolutely nothing to increase your gas mileage. We spoke to a fuel-systems engineer who works for one of the major oil companies. He said that because of all these rumors floating around on the Web, his company tested acetone in its own labs and found no increase in mileage. None. And he said the equipment is precise enough to detect anything over a 1 percent difference.

TOM: But it’s worse than useless ­ it’s also harmful. Acetone is the primary ingredient in nail-polish remover. And while it will burn and is a high-octane material, it’s also a very powerful solvent. So while it’s in your fuel system, it’ll be eagerly dissolving all of your rubber components … like gaskets and O-rings.

So, Eric, it seems that your friends are most likely self-deceived by poorly controlled observations – i.e. anecdotal experience. The acetone claims fail to pass both the plausibility and the evidence test. If you want to keep these friend (for whatever reason) or can’t make it to TAM6, I suggest you take the approach of presenting yourself as an open-minded skeptic. You simply want to know the truth. Try to keep them focussed on each point of logic and evidence, and don’t let them make ad hominem attacks or commit logical fallacies. Do what you can to diffuse any emotional content from the discussion – make it as abstract as possible. It may not work, but it’s worth a try.