You may have heard about the Fuelshark. According to the package, you may have seen it on TV. That same package also says the Fuelshark will save you 10% or more on gas, and all you have to do is plug it into your cigarette lighter. Too bad the Fuel Shark is bullshit. And it's $39.95 worth of bullshit.


Seriously, you could plug an olive into your 12V socket and get similar results to what the Fuelshark does. And most of our regular readers will know this. But there's still plenty of people out there who are bright people who don't know or care about cars, and they want to save money on gas. Who can blame them? They may Google the Fuelshark, and find videos like this one from a Pittsburgh ABC affiliate:

Which, of course, may be enough to convince them that this thing actually works. Hell, they even have a quote from a Jaguar mechanic! He's got to know absolutely everything there is to know about car electrical systems from his years of dealing with Lucas electrics.


The point is, there are still many people buying the Fuelshark. The company claims they've sold over 10,000 of them. The company also sent me two review units, out of the blue, which started all this in the first place. So, let's get to some real science here, and allow me to explain why I believe the Fuelshark is complete and total snake oil. With science.

I should also mention that Fuelshark isn't the only product like this out there. There're others that use essentially the same methods and make the same claims, like the Fuel Doctor, and the results I found here would apply to those similar products as well.

First, I did the best, most controlled test I could manage to see if the Fuelshark had any effect at all. They claimed that it would work as well on older, carbureted cars as newer ones, so this first test I did with my '73 Beetle.


In order to isolate the results as much as possible from environmental conditions, traffic, etc, I decided to do a test where I could maximize control and minimize environmental variations. So, I ran a fuel line from the fuel pump directly to a gas can with 1/4 gallon of gas, then started the car and let it idle.


By idling at a constant engine speed, I could time how long it took to drain that 1/4 gallon. One run with the Fuelshark, one without. I also turned the lights on and off at set times in the test to vary the electrical load on the engine.


Here's the results: without the Fuelshark, the Beetle ran for 44 minutes, 37.18 seconds. With the Fuelshark, the engine ran for 42 minutes, 20.51 seconds. Which means it actually did a little worse.

I don't actually think the Fuelshark made the engine use more fuel — I just think it wasn't doing anything at all, and the results I got were in the normal range of variability. In fact, the Fuelshark people suggest testing this way:

we find that actual road tests show the best results as the Fuelshark works best when driving conditions are varied by road grades, turns, start/stops/ engine cut off and restarts, and highway driving combined.


... which is pretty much the best way to introduce as many variables as possible into a fuel economy test, and would cause the most variation in MPG no matter what the car had plugged into it. You would have to do a massive amount of testing like this to get any remotely credible MPG results.


So, I saw no result on my real world test, but to really understand the Fuelshark, we need to try and figure out what it's actually doing. The company claims the Fuelshark works in two slightly different ways. On their website, they claim

To maintain optimum electrical performance and improve MPG, your car will run better with a stable voltage environment. The Fuelshark provides just that. It is a multi-purpose Voltage Stabilizer. Fuelshark's design instantly stabilizes your car's electrical system, resulting in better fuel efficiency and overall performance. By stabilizing your car's electrical system, one big benefit is that your engine's spark plugs deliver a strong even spark, resulting in better combustion. This can increase power and MPG, in addition to having cleaner emissions.


And you'll notice they're also claiming better power and emissions, too. On the package, they claim the Fuelshark works by

... supplementing the required voltage for devices such as lights, stereos, power windows, etc. By using the Fuelshark, your car's battery does not struggle to provide the additional power needed to operate these devices.


This second explanation, at least, is grounded in reality, as Mazda is using a system (a vastly more complex system) that accomplishes essentially that to remove electrical load from the engine for increased fuel economy.


With these explanations in mind, let's look inside the Fuelshark. I enlisted the help of my friend Tom Jennings for this, since he knows a great deal about automotive electrical systems.


The Fuelshark consists of a blue LED (similar to this), a 35v 1000uF capacitor (like this one), and a 4700 ohm resistor (like this), along with a fuse and the housing/plug assembly. Based on the explanations of how the device works — either "conditioning" the car's electrical system or storing energy in the capacitor to help take electrical load off the engine when needed, we could only conclude that there was no possible way these components could achieve anything remotely like that. The amount of energy capable of being stored in that capacitor is on the scale of 1x10^-6 too small to be of any possible use to assist the car's engine.

That capacitor is capable of storing about 0.0072 joules of energy. A gallon of gas, for example, has 132,000,000 joules of energy, and a capacitor like the one in the Mazda system stores around 25,000 joules. The Fuelshark capacitor is capable of storing enough energy to keep that LED lit (through that resistor, which dims the LED and lengthens the time significantly) for 20 seconds or so, and that's it.


This device physically CAN'T do any of the claims the packaging or advertising suggests. It's not a matter of test procedures, the hardware is simply incapable of storing even a millionth of the energy required to perform the task the explanations describe. There's no hedging here — anyone with a basic background in electronics would know this.

I sent this to the president of Fuelshark, and got this response back:

Over the past four years we have sold over 10,000 Fuelsharks and monitored our returns and customer feedback vigorously. To date we have had 86 returns and I have personally spoken to dozens of customers that have told me that while initially using the product they tracked their mileage very carefully and saw significant increases in MPG without changing their driving habits. We have a section in our FAQ that demonstrates proper consumer testing procedures which was sent in the package with every Fuelshark order. ... Capacitors are now being used by auto makers to store and release energy in order to improve MPG. Mazda has introduced this in their SkyActive I-Eloop system using a 25 Volt capacitor. We use a 35 Volt capacitor. Article attached (Nov27 Mazda).


There's a some interesting things in this response. Most notably is that he's comparing the Fuel Shark to Mazda's system. He makes that comparison sound like one the Fuelshark wins, because, as he correctly points out, the Fuelshark has a 35V capacitor and Mazda uses a 25V. The problem is it's a totally useless comparison.

Voltage isn't a unit of how much a capacitor can store — it's something closer to a unit of electrical pressure. And, your car runs at 12V, which is why Mazda is stepping down their 25V to 12V-14V. Voltage just doesn't matter here.


The relevant measurement here would be a unit called farads. Mazda is using a capacitor in their i eLoop system that's rated at 120 farads. The Fuelshark' capacitor is rated at 1000 microfarads. If you do the conversion, you'll see that the Mazda capacitor is 120,000x larger than the one in the Fuelshark, which is why Mazda's getting the results they're getting.


Let's just think about that for a second. Even if you had a thousand Fuelsharks, you'd still need 120 times more to equal the ability to hold the same amount of charge as the one the Mazda uses. The Fuelshark simply does not have the hardware to do what they claim.

The fact that the Fuelshark people are comparing themselves to the Mazda system is amazing. In fact, if I had a hypothetical company that made plug-in capacitors for cars that allegedly saved gas — let's call it the FuelTorch — and I wanted to obfuscate and confuse people when asked how my system works, I would probably point out the voltage as a comparison. Just to confuse people. Because I would, fundamentally suck.


So, to recap, the Fuelshark not only does not give the results it claims, it physically, technically, can't give those results. It simply does not have the ability to store enough energy to be useful for anything beyond lighting a little blue LED. It's like peeing into the ocean and expecting the sea levels to measurably rise. It's not going to happen.

So, if anyone is Googling this, please consider these facts. If you still absolutely, positively must get some placebo to stick in your cigarette lighter, consider paying 1/10th as much for it.