A Volkswagen Passat TDI Clean Diesel achieved fuel economy of 77.9mpg on an 8,122-mile, 17-day tour of the 48 adjacent United States, which concluded this week at Volkswagenâs US headquarters in Virginia. Your mileage, of course, may vary â and likely by a considerable margin.  The Passat's miserly metering of diesel fuel resulted in a new Guinness fuel-economy mark for the category defined as "lowest fuel consumption – 48 US states for a non-hybrid car". While the category is narrow, the Passat's numbers are gaudy, battering the previous mark of 67.9mpg and topping the hybrid record for a similar journey by more than 13mpg.  Volkswagen spokesman Mark Gillies said the record-setting Passat's only modification was the replacement of standard-equipment tires with low-rolling-resistance models.  The Passat TDI was equipped with a 140-horsepower, 2-litre turbocharged direct-injection diesel and manual transmission. The EPA rates the car at 43mpg on the highway, and while its horsepower rating is modest compared to gasoline engines – the Passat's standard 2.5-litre engine generates 170hp without turbocharging – the diesel produces an abundance of torque at low speed, a characteristic of diesels and one that makes them enjoyable to drive.  At the wheel for the record attempt were Wayne Gerdes, a journalist who has set many fuel-economy marks, including the current 48-state hybrid vehicle record, and Bob Winger, an electronics engineer.  Gerdes is a self-described hypermiler, a driver who employs specialised strategies to achieve better fuel economy than would be possible in normal driving. Some hypermiler techniques are obvious: coasting as much as possible, accelerating gradually and carrying minimal extra weight.  Is one pair of socks sufficient for a 17-day journey?  But hypermilers take their pursuit well beyond the obvious. Speaking with BBC Autos, Gerdes said he used a scanning gauge to determine engine load at any given time of his trip. Engines tend to be most efficient at 70% to 80% load, so the scanner helped him remain within the optimal rev range by dictating how much or how little throttle pressure to apply.  "In steady-state driving, run at the lowest rpm possible for the conditions," Gerdes counselled. The manual transmission on Gerdes’ test car helped significantly there, as he could easily keep the car in the selected forward gear.  Among other seemingly counterintuitive chestnuts, Gerdes said to drive “as if you don’t have brakes”, which encourages a driver to coast toward a stop as much as possible.  His advice was not limited to deceleration. “Think about how traffic ebbs and flows,” he said. “Allow the car in front of you to accelerate ahead. I call that ‘expanding the bumpers’.” And he said drivers should not be afraid to adopt a trucker tactic on uphill sections: get in the right lane.  “When you're climbing mountains, you're going to get hurt. Most people think they can do it on cruise control. Don't. Do what the trucks do. Get in the right lane and go up at 25 to 30mph. You can get 20mpg going up, and then going back down is free. It's just a few minutes. It doesn't hurt your time. Get in the truck lane with your emergency flashers on.”  Steady as she goes, in other words.
By keeping a close eye on the tachometer, Wayne Gerdes achieved a 78mpg average on an 8,100-mile swing through the US.