It's not an official government rating, but it's pretty impressive nonetheless: 78 miles per petroleum gallon (MPPG) in a Volkswagen Passat TDI diesel.

It was anointed a Guinness World Record, so it's more than your neighbor bragging about his eco-abilities.

The team of auto blogger Wayne Gerdes and electronics engineer Bob Winger ran up 8,122 miles, using 104.94 gallons of fuel, covering the 48 contiguous states in a Passat diesel built at VW's Chattanooga factory and delivered for the drive as it came off the line, VW says.

VW itself set the previous record for a Passat diesel, 67.9 mpg in 2009.

The U.S. government's official highway mileage rating for the Passat diesel is 43 mpg.

The Passat TDI drive began June 7 from VW's U.S. headquarters in Herndon, Va., and ended when the team returned there June 24.

"Obviously, we employ some specialized techniques to achieve such figures, but there's no reason why owners of TDI vehicles shouldn't be able to achieve great mileage," Gerdes says.

A key tactic: Anticipate conditions. "Look for impediments or topography 15 to 45 seconds ahead, rather than reacting to where you're currently driving," he says.

Equipment on the mileage run:

•Shell ultra-low-sulfur diesel fuel.

•Continental PureContact with EcoPlus Technology tires, which have low rolling resistance. Those aren't available as a factory option on Passat TDI, but are available from tire sellers and were approved by Guinness.

•Linear Logic ScanGauge II to precisely measure fuel economy

ETC has contacted the team of Wayne Gerdes and Bob Winger with the intention of setting a new record for MPPG from our biodiesel.