This has dropped the Prius’s EPA down by 25 percent to an average of 45mpg.

The Prius costs an average of $3.25 per mile driven over a lifetime of 100,000 miles - the expected lifespan of the Hybrid.

The Hummer, on the other hand, costs a more fiscal $1.95 per mile to put on the road over an expected lifetime of 300,000 miles. That means the Hummer will last three times longer than a Prius and use less combined energy doing it.

The report, published earlier this year by CNW Marketing Research, Inc., is titled "Dust to Dust: The Energy Cost of New Vehicles From Concept to Disposal." It is said to measure in dollars and cents all the energy used in creating, building, operating and disposing of each vehicle over its entire lifetime. The report says a Prius costs $3.25 per mile to operate, versus just $1.95 a mile for the Hummer H3.

In May, a response from the Pacific Institute said of the CNW report, "The little supporting evidence that it has released suggests that the contentions in the report are, at best, unproven, and are likely wrong: the result of faulty analysis, untenable assumptions, manipulation and misuse of facts and data, numerical mischaracterization, and inadequate review."

Because of its remarkable claims, however, the CNW report has been circulated widely and quoted in the media, continuing to spur questions from consumers.

Vehicle life and lifetime miles
For example, the Prius is assumed in this report to be driven only about 9200 miles a year, and to have a service life of less than 12 years and 109,000 miles. CNW assumes the average Hummer H3 will travel 207,000 miles over a lifetime of 15-plus years. The H1, incidentally, is assumed to have a lifetime of nearly 35 years, with 379,000 total miles.

According to data from the New Vehicle Consumer Study by Maritz Research, Prius owners plan to drive their vehicles an average of over 14,600 miles per year. And if Prius hybrids last only the 12 years assumed by CNW Marketing Research, Inc. -- also an unsupportable assumption, given the substantially greater service lives of other Toyota vehicles -- they would still rack up more than 175,000 miles in their lifetime.

Production costs versus operating costs
Using another puzzling assumption, the CNW report seems to assign the majority of a vehicle's lifetime energy input to its design, development and production. We turn again to the Pacific Institute, which cites five independent studies showing that the operation of a vehicle is responsible for at least 73 percent and as much as 90 percent of its lifetime energy cost.

One of these studies, from the Argonne National Laboratory, is quoted as concluding that "around 74% of all hybrid and internal combustion vehicle energy use comes from the operation of the vehicle."

Pacific Institute quotes the MIT study "On the Road in 2020" as saying that a comprehensive life-cycle energy analysis found that "80% to 90% of all energy was used in the operation stage, 7% to 12% in the materials production stage, and the remainder in vehicle assembly, distribution, and disposal."

So, with lifetime energy cost divided by lifetime miles, as in the CNW report, Prius's per-mile energy input would be substantially better than is shown in that report, and better than that of a Hummer.

Our method of measurement
Toyota, meanwhile, employs its own standard to measure each new vehicle's environmental "footprint." Toyota's Eco-VAS, or Eco-Vehicle Assessment System, is a tool for predicting and reducing the environmental impact of its vehicles. It also measures the energy required in a vehicle's production and operating stages, but goes beyond analysis to provide a framework for design changes that improve environmental performance.

Toyota's engineers use Eco-VAS all through the process of design and development and leading to production -- measuring a vehicle according to six critical standards: fuel efficiency, exhaust emissions, external vehicle noise, lifetime environmental impact, recyclability and the reduction of substances of concern.

In design and production, these standards lead to the replacement of hexavalent chromium, lead and other substances of concern, lower the use of in-plant energy, and employ some plastic parts made from agricultural products rather than petrochemicals.

In the operating stage of a vehicle's life, Eco-VAS improves fuel efficiency, produces fewer greenhouse gases and smog components, and cuts the impact of batteries, tires, oil and parts used in maintenance.

And, in the recycling and disposal phase, Eco-VAS standards improve the collection and reuse of recyclable materials, provide for easier dismantling and prevent the production of harmful waste.

The bottom line
The people at Toyota are no strangers to environmental awareness and to the positive changes that benefit both our consumers and the world they inhabit. With Eco-VAS, we're taking giant steps to do even better.

And Prius owners, knowing the facts about the gentle environmental impact of their cars, continue to be proud of the choice they've made.