THE Volkswagen Jetta TDI rolled gently to a stop at the red light  no squealing brakes, no smoking tires. On the dashboard, the trip computer display read 62 miles per gallon.

For John Taylor, that was not very good.

Days earlier, Mr. Taylor and his wife, Helen, had completed a 48-state drive averaging 67.9 miles a gallon: 9,505 miles in 19 days, filling up only 13 times and spending a total of $371.03 for diesel fuel. That beat the Guinness record of 58.8 miles a gallon they set a year ago on nearly the same route in an almost identical 2009 Jetta TDI.

One difference this time was the tires. The Jetta was fitted with Goodyear Assurance Fuel Max tires, low-rolling-resistance models that will be standard equipment on the 2011 Chevrolet Volt.

Goodyear, a sponsor of the Taylors’ drive, says the new tire reduces rolling resistance by 27 percent compared with the conventional Assurance tire it replaces, which translates to a 4 percent improvement in highway fuel economy.