Diesels and hybrids.

Not to disparage the growing hybrid market in the United States, but I feel that diesels come out unfairly when the two are compared [see "Diesels versus Hybrids: Comparing the Environmental Costs," July/August]. I realize that hybrids are cleaner with respect to emissions but I don't think the EPA comparisons of gas mileage are accurate. In speaking with many hybrid owners, they are getting nowhere near the EPA estimated gas mileage, whereas many diesel owners, especially those using biodiesel, are getting significantly higher mileage than the EPA rating. Having just purchased a 2002 Jetta diesel (and still looking for a source of biodiesel), I know that I am getting around 50 miles to the gallon in mixed driving conditions.

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In addition, diesel motors tend to last longer than traditional petrol engines (notice the remaining Mercedes diesels still on the road). I think potential car buyers should be aware of the real world advantages of purchasing a diesel car.

JOSHUA POPPEL

New York Bicycling Coalition

Albany, New York, U.S.A.

It would be interesting to know if our government has a prejudice against diesel engines. Why else would the EPA put out such erroneous statistics on fuel mileage? Consumer Guide says the Prius got 42-45 mpg in "normal driving" in their tests, and as low as 36 mpg in "gas eating performance runs." This doesn't agree at all with the EPA numbers of 60 mpg in the city and 51 mpg on the highway.

If you talk to Prius owners, I don't believe many of them get the EPA advertised mileage. On the other hand, the EPA mileage for the diesel Golf/Jetta is ridiculously low. Again, Consumer Guide reports the Golf diesel with manual shift averaged 41.5 mpg, which is low if you ask people who own them. My wife and I bought a Golf diesel with manual shift a year ago and have kept meticulous records of our mileage, and for the first 18,000 miles we averaged 46.7 mpg. Other VW diesel owners have told us they average close to 50 mpg.

If your author goes back and recalculates the numbers for environmental costs using realistic numbers for mileage, instead of what appears to be propaganda put out by the EPA, the diesels would come out much better.

JAMES R. SAUER, M.D.

Minocqua, Wisconsin, U.S.A.

Thanks for another interesting issue. I might mention, however, that it has become widely acknowledged that the miles per gallon figures you give for the Prius are greatly exaggerated. For example, in the real world my neighbor gets 40 mpg in city driving. If the Prius could be plugged in to be recharged with household current overnight and had a switch to run on battery only, that figure could be much higher.

STEVE JUNIPER

Berkeley, California, U.S.A.

I was eager to download your article from the World Watch website, as I have wondered for some years now how much "healthier" gasoline-electric hybrids are for the environment and for the individual (if at all). The information in your article confirmed that my intuitive sense regarding diesels was correct. However, I was disappointed to read that assessing the environmental costs of hybrid battery production was "difficult to quantify." I accept that this is not a primary concern with respect to overall environmental costs, but some rough figures would have at least given some indication of significance. Also, you made no mention of the possible health issues associated with the close proximity of a relatively powerful electromagnetic field to the hybrid vehicle occupants (batteries in back, electric motor in front). I realize that this is also not easily quantified, but I strongly believe that it deserved mention nonetheless. Regardless, I very much appreciated the information that was in the article, and I intend to use this knowledge myself and to pass it on to those I know who are deciding on their next vehicle.

BRADLEI BORJEAU

Peterborough, Ontario, Canada

Author Corinna Kester responds:

The EPA fuel economy test was developed in the late 1960s. The city test averages 20 mph over 11 miles and has 23 stops, and the highway test averages 48 mph over 10 miles with no stops. Both are done in moderate weather without vehicle air conditioning. Obviously, these two specific tests cannot mimic the actual use patterns of all drivers, as conditions vary widely. The benefit of the EPA ratings is that fuel economy is determined using a standardized methodology that enables comparisons among vehicles. Many automobile rating companies do not systematically test fuel consumption, making it difficult to compare mileage reliably.

One reader suggests Consumer Guide for fuel economy ratings. Their numbers are as follows:

Prius, 42.6-45.2 mpg; Civic Hybrid, 31.8-39.3 mpg; Accord Hybrid, 26.2-29.0 mpg; Golf Diesel, 41.5 mpg; Jetta Diesel, 38.5 mpg; Passat Diesel, 28.7-35.3 mpg. These data suggest that these diesels and hybrids are roughly equivalent in terms of fuel efficiency. This makes a much stronger case for a diesel purchase today. Consumers may want to wait until new air pollution rules in 2007 reduce the sulfur content of diesel fuel, making a diesel purchase a winner in all categories.