Fuel Economy Concerns Influence New SUV Design

An honest concern for the environment requires an open mind and willingness to accept counter intuitive notions. Nature, after all, isn't always logical. The same goes for human endeavors ... although sometimes they can surprise and inspire us.

Take, for example, General Motors and its emphasis on full-size platforms for the rollout of its advanced two-mode gasoline-electric hybrid system. It's easy to assume that since this is the latest high-profile hybrid technology, the application should be a Prius-like small car platform to maximize impact. In an environment of historically high fuel prices, a super-high efficiency small car makes news and, to most people, makes sense.

But then, the reality: Not everyone wants to drive a small car. Needs and desires differ, which is as good an explanation as any to illustrate why there are so many different sizes and configurations of vehicles on the market. We like our freedoms, and one of those is the ability to choose the vehicle that suits our needs, both functionally and aesthetically. Plus, there are numerous small- to mid-size cars already available that consumers can buy to achieve their fuel efficiency goals today. Few larger platform vehicles fit that criterion.

One hard and fast rule of fuel economy is that heavier vehicles take more energy to move down the road and thus consume more fuel. Reducing weight will improve both fuel economy and vehicle performance. But with all the additional comfort and safety equipment that goes into today's vehicles (some of the latter federally mandated), taking weight out of a vehicle is a major engineering challenge. Interestingly, the average weight of light trucks dropped in both 2005 and 2006, while the weight of cars increased slightly. Light trucks, including SUVs, must by design be heavier to handle additional loads.

In recent years, consumers have overwhelmingly made SUVs one of the top vehicle choices. Overall, these vehicles have offered little in the way of fuel efficiency. Is all lost if you need the passenger capacity and functionality of a vehicle like an SUV or crossover but really want to drive "greener?"

The recently-released Environmental Protection Agency (EPA) report, "Light-Duty Automotive Technology and Fuel Economy Trends: 1975 through 2007," points out some key facts to consider. The trend toward light trucks (a segment that includes SUVs, vans, and pickup trucks) displacing traditional car sales grew to 50 percent of the total under 8,500 pound gross vehicle weight rating (GVWR) sales in 2002. After nearly 20 years of steady growth, primarily from the popularity and proliferation of the SUV segment, the light truck market leveled off and has been fairly constant for the past six years.

Sport utility vehicles accounted for just 10 percent of the total light vehicle mix in 1990. Significant growth in that segment now has SUVs accounting for 30 percent of the total U.S. market. Growth of the SUV market, however, peaked in 2003 and has been comparatively stable since.

There is a small but vocal minority these days who believe that larger personal-use vehicles have no place on our highways. This is a societal issue that will unfold over time. It is not a fuel economy issue. While larger vehicles do require more power to move down the road, they also offer much more functionality including the ability to accommodate larger families, tow recreational trailers of all types, and haul heavy or bulky loads. Thus, they have their place.

Light trucks, the segment that includes SUVs, generally deliver lower fuel economy than cars. EPA figures show that the light truck segment's fuel economy average is 5 to 7 mpg lower than that of traditional automobiles. Plus, there's the issue of CO2 greenhouse gas (GHG) emissions. The EPA report underscores that fuel economy is directly related to CO2 emissions and that light-duty vehicles contribute about 20 percent of all of the country's carbon dioxide emissions, which makes this a real issue.

If we really want to reduce consumption of gasoline and minimize motor vehicle-related CO2 emissions, then it's crucial to follow a strategy that promises to have the greatest chance of success and achieves these goals quickly. This means that alongside environmentally friendly small car advancements, we also must make room for important fuel efficiency advancements in high volume mid- and full-size vehicles that consume the highest percentage of fuel and generate the greater CO2 emissions.

Big SUVs, like the Chevy Tahoe, clearly have the most to gain. A small increase in economy and accompanying reduction in GHG emissions at the low end of the fuel economy spectrum offsets much larger improvements on the higher end from the most efficient vehicles. The two-mode, however, does not just provide a small increase. It brings a huge improvement of overall efficiency that compounds the equation.

The Tahoe Hybrid delivers a 50 percent increase in city driving compared to the standard 5.3-liter non-hybrid version. Since the two-mode system also offers assistance at higher speeds, highway and combined fuel economy numbers are improved as well. The amount of fuel saved with the use of this hybrid system is dependent on how much stop-and-go city driving is done in a daily routine. Many large SUVs are used as family shuttles and spend a great deal of time in the city cycle, running errands and moving kids from one activity to another. In this environment, the fuel economy improvement is truly substantial at about 50 percent.

Mix in some highway driving and the percentage moves somewhat downward. EPA calculates its combined fuel economy estimates by factoring 55 percent city and 45 percent highway driving. The combined fuel economy number for the two-wheel drive Tahoe Hybrid with its 6.0-liter V-8 two-mode system is 21 mpg, a 30 percent improvement over the standard Tahoe with a smaller and less powerful 5.3-liter V-8. At 16 mpg, a conventional Tahoe would consume 937 gallons of fuel to drive an annual average of 15,000 miles. The Tahoe Hybrid would use 714 gallons over the same distance. That's 223 gallons of gasoline saved with just one vehicle in one year.

To put this in perspective, consider the fuel economy improvement achieved by the Honda Civic Hybrid over the standard four cylinder Honda Civic. The standard Civic delivers 2008 EPA numbers of 25 mpg city and 36 mpg highway, for a combined average of 29 mpg. The Civic Hybrid achieves 40 mpg city and 45 mpg highway with a combined average of 42 mpg. The spread between the two is 13 mpg, which appears to be a much greater fuel savings than the Tahoe Hybrid. But when you factor in annual fuel consumption for the two vehicles, the Tahoe Hybrid actually saves much more fuel. At an average of 29 mpg, the standard Civic requires 517 gallons of gas to go 15,000 miles. The Civic Hybrid, at a 42 mpg average, covers the same 15,000 miles on 357 gallons, saving 160 gallons of gas each year. The bottom line: The Tahoe Hybrid saves 63 gallons - or 39 percent - more fuel each year than the Civic Hybrid.

In addition to the fuel savings, consider the impact on the environment that this big jump in fuel efficiency brings. A powertrain advancement that reduces fuel consumption by 223 gallons per year without sacrificing functionality also reduces CO2 emissions by 4,363 pounds annually - more than two tons. If efficiency improvements like these were applied to all new full and mid-size SUVs the potential fuel and emissions savings would be enormous. Just do the math.