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How IntelliChoice Real MPG works

Science and the real world intersect to create brand-new MPG measurements

The ritual at the new-car dealership lot is pretty much universal: the prospective buyer circles the car to appraise its styling. Eventually, he or she will crane their face close to the window sticker to analyze its price and options. And finally, they’ll gaze at its EPA city and highway numbers – with a questioning expression. Will the car of their dreams really deliver on those numbers?

The answer to that is a very qualified yes. They’re a little like a lottery drawing: sometimes they’re right and sometimes, well, not so much. Feel lucky? True, since the EPA’s most recent revision to its system in 2008, they certainly provide genuinely useful guidance. But actually getting those results famously depends on ‘how’ you drive (and sometimes to a surprising degree). Moreover, these numbers are also – every now and then – infamously wrong.

How could that possibly be, you ask? The vast majority of the city and highway numbers you see printed on those window stickers are actually generated by the car companies themselves; only a small sampling are spot-checked by the government which itself has been historically ill-equipped to handle the entire undertaking. In other words, it’s a largely self-policed system. And while no one should suggest that the car company’s testing is ever deliberately dishonest, it’s an unwieldy structure where mistakes happen. Plus, it’s simultaneously riddled with perfectly legal loop-holes that are sometimes a little too irresistible not to exploit. Bottom line: plenty of new-car buyers are legitimately queasy with this sole source of mileage information.

Which is why we believe the marriage between IntelliChoice, MotorTrend and our partner, Emissions Analytics can provide real-world mileage information which we’ve appropriately named IntelliChoice Real MPG.

In short, our test cars are equipped with an extraordinarily sophisticated and sensitive array of sensors to record their mileage over a carefully-considered course that realistically represents both city and highway driving. Preparing each car requires about 90 minutes; the test itself takes another three hours, and that’s followed by painstaking analysis of the results.

That’s the thumbnail version. More specifically, Emissions Analytics’ sophisticated electronics sniff once-a-second exhaust samples for CO and CO2 to calculate the fuel’s burn-rate by multiplying it by the total exhaust flow (also measured). The plumbing to do this delivers the exhaust to the flow-rate analyzer (mounted on the car’s rump where it’s tucked out of the airflow) then threads an exhaust sample of it inside the cabin to a gas-analyzer (where the sample is heated, dried, and lit by an infrared lamp causing molecules of CO2 and traces of CO and hydrocarbon emissions, to absorb particular, measureable wavelengths).

A mini weather station monitors environmental factors; a GPS data-logger notes speed and position; there’s even a separate electrical power supply (a very big battery) to isolate the system from imposing parasitic losses on the car. Typically, each car has between 1,000 to 5,000 miles on the odometer, are warmed before testing, are set in the engine-on default condition (not put into 'eco' or 'sport' modes), and are carefully ballasted to a consistent test weight to reflect partial loading of the car with passengers and luggage. Moreover, both before and after every test, the analyzer is checked, first, with a flow of nitrogen (as a ‘zero’) and then one containing precisely 16% of CO2 to check its calibration.

The test is 140 minutes long and encompasses a wide variety of road conditions from residential, city, arterial and highway. Each element is repeated a number of times. The cycle was defined having studied typical US driving patterns, and with the overall total test being 90 miles long, using average city speeds of 22.5mph and highway speeds of 65mph.

But as important as any of this is what we pour into the car’s tank before each test. You’ll probably be surprised to learn that the energy density of gasoline can vary by 4% or so between deliveries of what’s outwardly the ‘same’ fuel to the same gas station. That’s an intolerable variable for mileage testing. Which is exactly why the EPA, as well as other premier testing facilities, use a consistent ‘test’ fuel. Here, we’ve partnered with the good folks at Chevron who, at their Richmond refinery in the Bay Area, have brewed up for us a trio of standardized, real-world ‘typical’ fuels (regular and premium grades with just under 10% ethanol, and of course, diesel) with a known carbon content, and that’s consistent and will remain chemically stable for years. To ensure consistency, Chevron captures a gallon per every barrel for periodic post-analysis testing (similarly, our local partner, Dion & Sons, quarantines an additional quart from every barrel, and stores it in a protected environment) before delivering to our facility.

The test route is a carefully crafted, 88-mile drive along a nearby freeway, through a myriad of city streets (its design influenced by the EPA’s city cycle) plus connector roads to illuminate the influence of elevation changes. And there’s plenty of cleverness woven into its planning. The freeway portion (with a target average speed of 65 mph) is traversed six times (two with the A/C set on max, four with it completely off). In between, the city portion, with 22 turns and conducted at an average speed of 22.5-mph, is repeated four times, twice each at those same extremes of climate settings. The repeats are for two reasons: to check for internal consistency but also to quantify the impact of climate control on mileage–which can be quite substantial. To cope with hot days, our Emissions Analytics team has cool vests at the ready.

A basic difference with EPA testing is that our Real MPG results are produced in the Real World with actual aerodynamics, true rolling resistance, and real-traffic acceleration rates. Not on a dyno with simulated aerodynamics, and clever test drivers following a wiggly line on a monitor that their right feet could reenact in their sleep. (The voracity of which was even questioned by the EPA’s Director of Transportation and Air Quality who recently fretted that the dyno jockeys were getting a little too effective at tracking the traces).

Though we strive to drive the cars in a rigorously consistent manner, the unavoidable variations that occur in traffic are corrected for by modeling the data to see how the second-by-second CO2 varies with speed, acceleration, gradient (and other factors). Small corrections are then made whenever the real driving deviates from our target test cycle, and if a profile differs too much, it means retesting. However, the golden lining here is that these smaller corrections give us valuable insight into how each vehicle reacts to changes in key driving behaviors (for instance, acceleration rate). In addition, for hybrids, we note their pre-and-post-test state-of-charge and use this to further fine-tune their final Real MPG numbers.

All this, plus the insights we glean about the impact of air conditioning use, allow us – or rather you, via selection sliders – to fine-tune a car’s mileage result to better reflect your specific driving style and driving environment (our unadjusted, Real MPG results are weighted 55% city and 45% highway –per the EPA’s practice.)

So if you’re one of those who’s ever had that 'doubtful feeling' while staring at the mileage numbers on a window sticker (and who isn’t) welcome to our new real-world alternative – IntelliChoice Real MPG.


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