"Our bus runs on biodiesel fuel. This clean-burning alternative to gasoline is produced from renewable - and domestically grown - resources. Biodiesel contains no petroleum, is biodegradable, nontoxic, and essentially free of sulfur and aromatics..."Excellent!
"It does cost more than regular diesel, but consider this: The Google shuttle carries an average of 155 employees a day. Each run totals about 75 miles - that's 11,625 miles a day we're not driving. If the average car gets 25 mpg, then we're saving some 465 gallons of gas a day, or 2,325 gallons a week - weekly savings of $4,998.75 (figuring $2.15/gallon)."
(disclaimer: I work on the Blogger team at Google, but alas can't take advantage of this wonderful service because I don't live in SF...)