Beyond turbos, what car technology works to boost mpg?

Consumer Reports News: March 22, 2013 12:38 PM

The past few years have shown dramatic fuel economy improvements in all types of cars in our testing. Rather than exotic technology, such as hybrids and vehicle electrification, most of the improvements have come the old-fashioned way: through conventional technologies applied to gasoline engines.

Following our reports on turbocharged engines and hybrids that don't live up to their fuel economy claims, we decided to take a look at the technologies employed by cars that did show significant fuel-economy improvements in our testing. Among them: direct injection, continuously variable transmissions, conventional automatic transmissions with multiple ratios, and electric power steering. Some models have added more than one of these technologies simultaneously, most typically during redesigns.

Visit our guide to fuel economy to find the most fuel-efficient cars by class and learn to save gas in your current car.

We have collected examples with fuel economy data from our tests to illustrate the potential benefits from various technologies, understanding that in some cases, there may be other subtle changes at play. Here's what we found:

Continuously variable transmissions
CVTs have proliferated in recent years. They boost efficiency by keeping the engine working at its optimum rpm for power and fuel economy at all times. While some CVTs accentuate engine noise obnoxiously, we've found that not all of them do. And based on our testing, they can deliver significant improvements in fuel economy and improve acceleration, as well.

The Subaru Legacy, Outback, and Impreza, as well as the Nissan Sentra and Versa, and Honda Accord switched to CVTs with their latest redesigns. Gas mileage improved 3-5 mpg over their predecessors. Acceleration was also quicker in all but the Versa. With the Impreza, the CVT allowed Subaru to use a smaller engine and gain 3 mpg.

More gears
More automakers have recently been introducing conventional automatic transmissions with six, seven, and eight speeds. Two automakers are even reportedly set to introduce models with nine-speed automatics this year.

Looking at the Audi A4, which switched from a six-speed automatic to an eight-speed in 2011, showed a that that change alone boosted fuel economy by almost 14 percent, from 22 mpg to 25 mpg and slightly improved acceleration.

Direct injection
As the name implies, direct injection squirts fuel precisely where it's needed in each cylinder's combustion chamber in gasoline and diesel engines, rather than into the cylinder head outside the cylinder. This allows the engine to deliver the same power on less fuel. It's becoming common in all kinds of cars.

When Toyota added direct injection to the Lexus RX SUV in 2007, gas mileage increased by one mpg, even with a larger engine displacement. Plus, the powertrain upgrade produced significantly quicker acceleration. The same engine swap in the 2011 Toyota Highlander Hybrid helped boost fuel economy from an impressive 23 mpg for this large, 7-passenger SUV, to an almost sedan-like 27 mpg. The slightly larger, direct-injected engine was the only change in both SUVs.

Combining improvements
Several automakers have boosted fuel economy significantly by combining more than one of these technologies. The new four-cylinder Honda Accord gets an impressive 30 mpg by combining a smooth-and-unobtrusive CVT with direct injection and electric power steering. That figure marks a 20-percent improvement over the old Accord's respectable 25 mpg.

Several other automakers have combined direct injection with additional gears in their conventional automatic transmissions, including Hyundai, Kia, and Mazda. Mazda even has a name for this combination: It markets cars that use direct injection and the company's new six-speed automatic (up from five) as SkyActiv models. When Mazda added the SkyActiv powertrain to the Mazda3 for 2012, the combination boosted mileage by 14 percent and improved acceleration.

Kia did the same with the Soul for 2012, gaining 1 mpg and significantly improving acceleration.

Other technologies
Electrically assisted power steering has virtually become the norm in the industry with significant fuel economy improvements. Most often electric power steering systems are introduced with complete redesigns of a vehicle. Thus, they're typically coupled with other technology improvements.

Looking at new models as technology is introduced can be a little misleading, because other factors can change during a redesign. For example, the new Honda Accord is slightly smaller than the old one, while the current Subaru Outback is significantly taller than the previous model. But we've tried to keep examples here as close to equivalent as possible. The Audi A4, Kia Soul, Lexus RX, Mazda3, and Toyota Highlander had no other changes.

Direct injection Overall mpg Acceleration (sec.)
2008 Toyota Highlander Hybrid 23 8.2
2011 Toyota Highlander Hybrid 27 7.6
2004 Lexus RX 18 8.8
2007 Lexus RX 19 7.3
Multi-speed automatic    
2009 Audi A4 22 7.3
2011 Audi A4 25 7.2
2008 Subaru Impreza 24 9.5
2012 Subaru Impreza 27 9.2
2005 Subaru Outback 21 11.8
2010 Subaru Outback 24 10.7
2010 Nissan Versa 27 10.3
2013 Nissan Versa 32 10.6
Direct injection+multi-speed auto    
2010 Kia Soul 25 9.8
2012 Kia Soul 26 8.6
2010 Mazda3 28 9.4
2012 Mazda3 32 9.1
2009 Hyundai Sonata 26 9.8
2011 Hyundai Sonata 27 8.2
2009 Hyundai Azera 20 7.2
2012 Hyundai Azera 23 7.2
Direct injection+CVT    
2011 Honda Accord 25 9.2
2013 Honda Accord 30 7.7

The fuel economy improvements are real. According to a University of Michigan study, the average EPA fuel economy of all new vehicles has risen from barely 20 mpg at the end of 2007 to 24.6 mpg in January 2013. But not all technology implementations make the promised gains, underscoring the importance of checking our test findings before buying—especially if hitting a high fuel-economy benchmark is a key motivator.

Eric Evarts


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