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Fact Check: How Accurate are those MPG numbers?

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On: Mon, May 13, 2013 at 2:01PM | By: Teddy Field


Fact Check: How Accurate are those MPG numbers?

A decade ago, gas was cheap and SUVs had their own zip codes. Americans didn't care about fuel economy. We just wanted big vehicles to traverse our big country, as we slurped on our Big Gulps. But that all changed when gas prices spiked in 2008. Customers were forced to abandon their 15 mpg ImpressUVs, and car makers were forced to start building vehicles that offered a similar experience, without sucking down $900 worth of gas every week.

After billions in combined R&D, automakers have been able to develop a bevy of fuel saving technologies, that have turned today's vehicles into fuel-sipping wonders. For example, SUVs like the 2013 Chevrolet Equinox can achieve 32 MPG on the highway, while providing a total of 63 cu-ft of cargo room, 1,500 lbs of towing capacity (the V6 can pull 3,500 lbs), and there's even a built-in mobile Wi-Fi hotspot. The Environmental Protection Agency (EPA) rates the 4-cyl FWD Equinox at 22 city/32 highway/26 combined.

By comparison, an equivalent 2003 Chevrolet Trailblazer (2wd / 6-cyl) could manage only 14 city/20 hwy/16 comb. So clearly, automakers have dramatically increased efficiency, along with content and capability. But just how accurate are those MPG figures?

Every new car sold today comes with a big sticker (Monrony Sticker) that lists fuel economy figures that the EPA claims to be possible. Three numbers are listed: City/Highway/Combined (obviously, a combination of both city and highway driving). These numbers are reached by subjecting the vehicle to a series of calculated actions, designed to mimic varying traffic conditions. Each vehicle is strapped to a dyno machine (essentially, a big drum in the floor that the drive wheels spin), then a trained driver sits in the car, accelerating and braking the car according to prompts shown on a screen in front of the car. The city test cycle was developed in 1972 to mimic rush hour traffic in downtown Los Angeles, and the driver has to cover 11 simulated miles, making 23 stops along the way.

Varying rates of acceleration and speed are used throughout the test, then they measure the amount of fuel used to complete the test cycle. To ensure consistency, the EPA has special batches of fuel made for the test vehicles to run on. Each vehicle then completes 4 additional test cycles, each putting different loads on the car's drivetrain. In one test, the test chamber is heated to 95 degrees, and the vehicle's air conditioning is used while the car accelerates (at times) to over 55 mph. The city test cycle is also performed again, except the temperature in the testing chamber is reduced to a frosty 20 degrees Fahrenheit.

The fuel consumption data is then plugged into a series of mathematical equations, and the car's official fuel economy rating is tabulated. But here's the kicker; the EPA tests only 15% of the new cars being sold in America. It's up to each automaker to perform these tests in-house, following the guidelines given to them by the EPA. The test data is then submitted for official certification. And to ensure that everybody follows the rules, the agency will 'audit' (perform the test in-house) 15% of the vehicles submitted each year.

With fuel economy becoming a bigger part of the modern customer's purchase criteria, having higher MPG numbers will obviously help to sell more cars. Although it doesn't happen very often, car companies have been caught inflating their MPG claims. Hyundai-Kia for example, were caught inflating the MPG numbers on 13 of their 2011-2013 models. Prior to the EPA audit, their 2011-2013 Hyundai Elantra had been rated at an astounding 40 MPG on the highway, and that made their little compact a big hit in the showrooms. Thanks to its coveted 40 MPG rating, Hyundai sold 202,000 Elantras in 2012. EPA testing then revealed that the Elantra was capable of only 38 highway MPG , but sales of Hyundai's (pretty decent) little compact remained strong.

The EPA's official fuel economy website allows owners to record the actual fuel economy that they're getting from a particular vehicle. The 2013 Hyundai Elantra, for instance, is officially rated at 28 city/38 hwy/32 combined. However, 19 Elantra owners have reported an average of 22 city/41 hwy/28.7 combined. The 2013 Chevy Equinox that we mentioned earlier, has an official rating of 22/32/26, but two owners have reported fuel economy numbers of 22/28/24.9. If you really want to dig up some dirt, the much advertised 47 MPG 2013 Ford C-Max Hybrid has been rated by 86 owners to only achieve 29 city/56 hwy/38.8 combined. Not bad numbers to be sure, but they're still a far cry from the official rating of 47 city/47 hwy/47 combined.

So what gives? Are they lying to us, or is it something else?

The main reason that 'real world' fuel economy is so much different has to do with how we drive. An EPA tech driving that Elantra in a climate-controlled lab isn't driving your morning commute. So he's not going to slam on brakes to miss that oblivious bike messenger. He's not going to floor it to pass that grandma driving her Town Car at 28 in a 45 zone. He's not going to run the seat heater, then forget to turn off the passenger seat heater when there's nobody in it. He's not going to drive with the windows down, and the music blaring (yes, electric accessories like seat heaters and stereos consume fuel). And lab tests don't account for heavy options like a full-glass panoramic roof, or the extra fuel consumption of manually shifting gears on a manually-shiftable automatic transmission. Plus, many would even argue that the EPA's testing procedures don't accurately reflect today's driving environment (the city cycle was designed in 1972, after all).

Between our natural behavior behind the wheel and (perhaps) slightly out-of-date testing procedures, actually achieving the advertised fuel economy is not just going to happen. You may have to modify your driving style, the vehicle might need to driven for a few thousand miles in order to break the engine in, or your Fuel Sipper XL just might not be capable of the claimed fuel economy.

But not all MPG ratings are on the low side. The EPA rated the diesel-powered 2012 Volkswagen Jetta TDI at 30 city/42 hwy/34 combined, but 10 owners have pegged the Jetta TDI's fuel economy at 35 city/48 hwy/39.7 combined. And many owners on the VW forums are claiming 50-60 MPG on the highway. So if you really want to know what kind of fuel economy to expect from a car, spend some time reading the owner forums. They'll be able to offer more insight on what it's like to actually live with a particular car.




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