With gas hitting a
national average of $4 a gallon, everyone's been talking about
how to improve fuel economy—myself included, be it for
taking summer road trips or avoiding goofy (and expensive)
gas-saving gadgets. But how do we know if the fuel economy we're getting is accurate, whether we're driving an unmodified car—or one equipped with a supposed miracle product?
On the road, most drivers just read the trip odometer and divide by the amount of gas they purchase. Simple enough, eh? Sure is, but that doesn't account for differences in the driving cycle during each individual tankful. And that can vary an enormous amount. Of course, over a few tankfuls, the variances will average out, right? Not exactly. Your vehicle will get poorer fuel economy during the winter. That's because of the increased electrical use for lights, wipers, heat and longer warmups—not to mention the extra drag caused by moving snow and slush out of the way of the tires. Spring and fall are good, but A/C use can certainly cause a mileage hit during the hot summer months.
In the back-and-forth
my weekly repair Q&A, one reader described to me his supposedly foolproof method for checking mileage—and subsequently the accuracy of his gas-saver gadget. It goes like this: He drove a delivery van on the exact same route twice a day. So he topped off the tank one morning, and drove the route like he normally does. He then installed a pretty pricey magnet (over $100) on the fuel line during his lunch hour, topped off the tank and drove the route again that afternoon. His mileage increased 12 percent, thereby proving his gadget worked. Right?
Well, even this sharp reader failed to account for the fact that in the cool morning temperatures, his fuel tank was also cooler and denser. It took a certain amount of fuel to fill the tank from the day before. And as he drove his route, the fuel heated up and expanded, largely because fuel pumps work continuously. Even then, only the excess fuel (heated up as it passes through the engine compartment) is returned to the tank. Consequently, it took a smaller amount of fuel to fill his tank. So he made the afternoon run with a warmer tank ... which he didn't refill until the next morning when the tank was cool. That cooler fuel had shrunk, making it appear that he had used less.
At my suggestion, our dear reader repeated the cycle with the magnet installed only during the afternoon runs. His results, it turned out, were very different. I had a feeling they would be.
But there's a whole lot more going on here. I'd argue that the mere
expectation of increased mileage will actually make your mileage increase. I've seen it happen a thousand times: If you know the fuel economy and a gas-saving gadget are under scrutiny, you'll accelerate smoother—and probably drive a bit slower, too. I call it “voodoo mileage.” To find accurate results, a driver must remain ignorant not only to how much fuel is being consumed, but also to the entire experiment. It would have to be a true double-blind study.
Here at PM, we report fuel economy numbers on our long-term fleet by averaging the economy over a period of some months, and from a number of our staff's driving experiences. When we do a comparison test, we'll drive all of the vehicles on the same day, at the same time, at the same pace, on a several-hundred-mile loop. We even swap drivers every half-hour to equalize driving styles.
There are some tech tricks you can use to help monitor your fuel economy as you drive. I'm fooling around with a couple of devices right now. I just installed a
Scangauge on my motorcycle. This $180 device plugs into the On Board Diagnostic System (OBD II) port under the dash of virtually any post-1996 car or light truck. It operates as a scan tool (much more on that here), so it gives me trouble codes and streaming data, but it also works as an electronic gauge cluster and trip computer. I can track battery voltage, coolant temp and sundry while tooling down the interstate. But one feature is invaluable: the instantaneous fuel economy readout. It gets fuel quantity data from the injection timing—the longer the injectors are open, the more fuel they squirt. It's amazingly accurate, too.
And I've just started tinkering with another gadget with some similar features: the
CAMP2 from HKS. It's a scan tool/gauge package/trip computer like the Scangauge, but it uses either the car's internal dashboard display or an aftermarket TV screen of any sort. It's intended to be professionally installed, but I've made a portable enclosure that I can suction-cup to the windscreen with RAM mounts. Unlike the Scangauge, the CAMP 2 has a graphic display that can be configured to reflect a dizzying number of parameters available from the vehicle's OBD II system. Input the car's weight, and you can even get an instantaneous horsepower indication. You can look at raw numbers, or at a simple analog-style gauge with a moving needle. But my favorite is the display that gives you something looking more like an oscilloscope trace, showing you what any given engine parameter is for the last few seconds or minutes—including rewind capability, so you can review after you've pulled over to a safe place.
If you'd like to alter your driving style to achieve high fuel economy, these devices are unparalleled. The slightest upgrade, downgrade or movement of your right foot leaves a dent or a bump in the economy trace. I've only begun to investigate its potential, so stay tuned.
Don't feel like dropping hundreds of dollars for one of these high-tech gadgets? Pick up an old-fashioned vacuum gauge at the local parts store. Monitoring manifold vacuum as you drive around will give you a fairly clear picture of your instantaneous fuel economy. BMW models have had a vacuum gauge integrated into the instrument panel for generations—it's simply labeled in miles per gallon instead of inches of vacuum. Higher manifold vacuum means higher mileage—the needle will sink alarmingly as you open the throttle, which will soon teach you to featherfoot. That's good advice, because altering your driving style is probably the least expensive and most effective gas saver of all.
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