We have enjoyed this top-notch analysis from The Energy Blog

First, The Coal Burning Power Plants

The average Coal burning power plant emits 2.17 pounds of CO2 for every kilowatt-hour it produces (EIA). Now, coal makes up only 48% (Colorado is around 70%) of our national electrical generation. Natural Gas produces 20% and it also emits CO2 (1.4 pounds per kWh). But, 30% comes from nuclear and renewable sources. So, taken together, our national electrical supply generates, on the high side, 1.51 pounds of CO2 per kWh.

Note: According to Xcel Energy in Colorado, this conversion is 1.6 pounds of CO2 for every kilowatt-hour, according to Xcel's mix of fuel generation (HC3, Summit County baseline data, 2009).

Now for the Cars

Basic Chemistry tells us that burning 1 gallon of gasoline emits 19.4 pounds of CO2. It also tells us, based on BTU content, there are 36 kWh of energy in that gallon of gas.
One place where all the research gets bogged down is trying to account for the varying efficiencies of our automotive contraptions. Automotive IC (Internal Combustion) engines are anywhere from 15 to 28% efficient in average driving. Well, for our worst case, empirical study, I am going to assume the most efficient – 28%. The most efficient means it is the most work we get for every globule of CO2 emitted. Indeed, the meaning is that 28% of the BTUs in that gallon of gas will get turned into useful, mechanical work. All the rest are wasted as heat. There are 124,000 BTU’s, give or take, in a gallon of gas, so at 28% efficiency 34,720 of them or 10.16 kWh, are actually used for something productive. Yet, we still created 19.4 pounds of CO2 burning that gasoline. A little math, and the emissions result for our car is 1.90 pounds of CO2 per kWh of work produced. Note that at the other end of the scale – at 15% efficiency - those numbers would be 5.4 kWh of useful work, and 3.59 pounds of CO2 per kWh.
Although this article is not about CO2 per mile (remember, simple), for the sake of reference, both the Chevy Volt, and my Chevy Malibu use approximately .240 kWh for each mile at a </i>steady<i> 60 Miles Per Hour. The Tesla Roadster claims to use .217. The Prius is about .220.

The Answer Please

The results show that burning gasoline in an IC engine powered car creates anywhere from 21% to 58% more CO2 than getting the same amount of energy from our electric grid for the same size and shape car. And, that point is important.

The other way many of these studies tend to confuse the issue is trying to relate everything to mileage – like pounds per mile. That is truly putting oranges in an apple barrel. The matter of mileage is irrelevant when comparing similar size and shape vehicles where the only difference is the source of power. They will both use the same amount of energy, which here is expressed in Kilowatt-hours. Regardless of whether they are powered by electricity, gasoline, or soda pop. The difference lies in the source of the power.

It should be clear by now that by replacing a gasoline powered IC engine vehicle with an electric one, for the same amount of power at the wheels, we would reduce CO2 GHG emissions by anywhere from 21% to 58% - even with our existing electrical supply. And, in fact, those who have bothered to reduce some of the complex, often politically motivated and artistically spun, studies down to their basic conclusions have found exactly the same thing.

Check out this great analysis from: The Hybrid Center

In an environment where American consumers are increasingly more concerned about the ecological, economic, and energy-related impacts of their travel, hybrid vehicles have garnered the lion’s share of recent attention as a near-term solution to those challenges. Yet when it comes to fuel-efficient drivetrains, hybrids are not alone. U.S. interest in diesel engines for passenger vehicles has seen a recent resurgence, in part because of creative automotive engineering. A number of the performance and emissions pitfalls that plagued diesel technology over the years have been addressed, bringing the latest round of what automakers call “clean diesel” vehicles a newfound respect among automotive enthusiasts.

Some of this new attention is warranted. Today’s diesel cars are meeting emissions levels that—for a diesel vehicle—would have been hard to imagine a mere decade ago. Like some of their hybrid counterparts they also offer impressive fuel economy compared to many conventional vehicles on the road. Such information has prompted some to speculate about whether today’s diesels might be deserving of the eco-friendly mantle currently held by efficient hybrids like the Toyota Prius and Honda Civic Hybrid.
So how do today’s diesels fare compared to today’s most popular hybrids? This edition of the Hybrid Watchdog takes a closer examination of the issue, and identifies exactly which parameters to consider when making such comparisons. Let’s dig in…

Under the Hood: Diesel Fuel Economy

One of the biggest draws of diesel technology is their impressive fuel economy. Diesels achieve higher mpgs (miles per gallon) due to two main factors. First, the compression ignition combustion process that occurs inside diesel engine cylinders is more efficient than the spark ignition process that occurs in gasoline engine cylinders, wasting less energy as the fuel is converted into mechanical motion.
Second is the fact that on a per-gallon basis, diesel fuel contains about 13 percent more energy than gasoline. This higher energy density helps push the vehicle farther down road, getting it—quite literally—more miles per gallon. As we’ll see shortly, however, the impressive fuel economy numbers of diesels don’t tell the whole story when it comes to assessing other attributes such as global warming, or oil dependence performance, or air pollution.

Global Warming Pollution

When comparing the global warming performance of two vehicles, most consumers simply compare their respective fuel economies. While there are other secondary global warming pollution factors for autos, the pollution a vehicle puts into the atmosphere is largely proportional to the amount of fuel the vehicle burns. Generally, the more fuel-efficient a vehicle is, the better it is from a global warming standpoint.

While this is a good general rule, it is important to note that using fuel economies to compare the global warming performance of two vehicles is only applicable when the two vehicles operate on the same fuel. Different fuels have unique concentrations of carbon, in a similar way that, say, different types of soda contain different amounts of sugar. This “carbon content” affects how much CO2 is emitted into the atmosphere when the fuel is burned.
Diesel fuel, in addition to containing more energy per gallon than gasoline, also has comparably higher carbon content. On a full fuel cycle basis (that is, incorporating not only the emissions at the vehicle itself, but also the emissions related to the extraction, refining and transporting of the fuel from wellhead to pump), the burning of diesel fuel emits roughly 13 percent more CO2 per gallon of fuel burned than does burning a gallon of gasoline. This characteristic of diesel fuel is why using a fuel economy comparison between gasoline and diesel vehicles to determine global warming benefits can be misleading. If a diesel vehicle consumes 25 percent less fuel than a gasoline powered vehicle, for example, its global warming benefit would in reality only be about 15 percent better than the gasoline vehicle.

Oil Dependence

A similar story holds when it comes to oil dependence. According to the joint U.S. EPA-DOE website, www.fueleconomy.gov, approximately 13 percent more crude oil is required to produce a gallon of diesel fuel than to produce a gallon of gasoline.*   Thus, from an oil security standpoint, merely examining the fuel economy of a diesel vehicle doesn’t capture the full picture. Using our previous example, a diesel vehicle that consumes 25 percent less fuel than a gasoline-powered vehicle would, in fact, consume only 15 percent less crude oil than the gasoline vehicle.

Air Pollution

All vehicles sold today are tested in a laboratory for tailpipe emissions and given a certification level — a grade, effectively — that specifies how clean the vehicle is in the emissions of four major pollutants: carbon monoxide (CO), nitrogen oxides (NOx), hydrocarbons, and particulate matter (PM). Today’s “clean diesels” are certified to meet what’s known as the federal “Tier 2 bin 5” emissions standard, along with an equivalent standard in California commonly known as “LEV II”.
Meeting the “bin 5” standard is a significant achievement for diesels which, because of the diesel combustion process, produces high levels of NOx and PM in the engine cylinders. However, the “bin 5” certification is an exceedingly common standard for today’s gasoline vehicles. Most of today’s hybrids and even some conventional gasoline vehicles are considerably cleaner, certified to “bin 3,” “bin 2,” (the lower the bin number, the better for the environment) and even the near-zero-emission California “PZEV” certification. 

Tale of the Tape

So how do diesels fare against the efficient hybrids that currently hold the eco-friendly vehicle mantle? In short, hybrids still carry the day. While diesels offer impressive fuel economy and have made improvements to their smog-forming emissions performance, efficient hybrids offer comparable (if not better) fuel economy, while still besting diesels on both the smog and global warming pollution fronts. Below is a sample comparison between two similar vehicles, one diesel and one a gasoline hybrid, that illustrates this point. Note, there are no diesel and hybrid models that allow for a pure apples-to-apples comparison on every major attribute; we chose these models for their popularity and market comparability.

Comparing the Environmental Performance of diesels and hybrids

Miles Per Gallon (MPG): The mpg ratings for the civic hybrid are about 13% better than the diesel Jetta in highway driving (45 versus 40 mpg) and 38% better in city driving (40 versus 29).  This reflects the benefits of hybridization in city driving and the strong fuel economy performance of diesel engines at sustained highway speeds.
Overall, the hybrid Civic (42 mpg) out-performs the diesel Jetta (33 mpg) in combined city and highway fuel economy by 27 percent. It should be noted that a vehicle’s fuel economy is highly dependent on manufacturers design decisions.  For example, the diesel Jetta could have a better mpg, but the vehicle designers chose to achieve higher acceleration rather than better fuel economy.

Global Warming Pollution: The mpg values help you determine how far you can go on a gallon of fuel, but more information is needed to accurately determine the global warming impact of a vehicle. Comparing the global warming emissions per mile, or carbon footprint, allows a direct comparison between two vehicles, while the EPA greenhouse gas score shows how a vehicle relates to the rest of the fleet.  

Carbon Footprint: The carbon footprint provides an estimate of the annual greenhouse gas pollution, measure in carbon dioxide equivalents (CO2-equivalent), from each vehicle model based on both the energy efficiency of the vehicle, the carbon content of the fuel being used, and driving 15,000 miles annually.

The Civic Hybrid emits an estimated 4.4 tons of CO2-equivalent per year, while the Jetta releases 6.4 tons per year, or approximately 45 percent more global warming emissions.

EPA GHG Score: While the diesel Jetta might appear to be a global warming laggard based on the previous comparison, the EPA GHG score puts these emissions into a broader context. The GHG score ranks all vehicles on a scale of 1 to 10. Both the diesel Jetta and Civic Hybrid are top scorers when compared to all other available vehicles in the showroom. The Jetta receives an 8 while the Civic Hybrid scores a 10. 

Oil Dependence: Comparison of petroleum consumption estimates show that the diesel Jetta consumes about 45 percent more petroleum than the Civic Hybrid.

Air Pollution: EPA also provides a similar score for comparing vehicles based on air pollution emissions.  The bin 2 certification earns the Civic Hybrid a 9 on EPA’s Air Pollution score, while the diesel Jetta’s bin 5 certification earns it a middle-of-the-road score of 6. 

Conclusion
Environmentally speaking, today’s diesels are a significant improvement over their recent ancestors and can offer notable fuel economy improvements over conventional gasoline vehicles. However, the exceedingly impressive tailpipe emissions and fuel economy performance of today’s most popular hybrids allow them to outperform their diesel counterparts. Nevertheless, diesels can offer global warming and petroleum reduction benefits compared to their conventional gasoline counterparts.

Many new models of diesel and hybrids will be entering the marketplace in the coming years with varying levels of environmental performance. It will be up to the automakers to decide whether these technologies are used to their full potential to reduce emissions, or whether they’re tapped to improve vehicle power and performance. When comparing diesels and hybrids, consumers should just be aware that “performance” in the categories of air pollution, global warming pollution and oil dependence are not merely a function of the vehicle’s fuel economy, but rather also a function of its emissions control system and even of the fuel’s chemical makeup.