Several readers have pointed out the apparent contradiction of my environmental road trip in a car that burns oil, even if I did drive a hybrid. And they’re right. Over two weeks I clocked 2,200 miles and filled up five times. That’s just over 50 gallons of gasoline, or enough to fill your entire bathtub.

I wanted to find out a little about my vehicle’s impact, so I stopped at Volkswagen’s emissions test center in Oxnard, California, just south of Santa Barbara. One note of disclosure: Volkswagen is a sponsor of Change Reaction, but the decision to visit the Oxnard facility was National Geographic’s.

Many car makers do their emissions testing in California for a simple reason: “California has the most stringent