As you know, GM and Ford offer higher-efficiency models of their full-size pickups, the Silverado/Sierra and Tahoe/Yukon XFEs and the F-150 SFE. All boast EPA ratings of 15 mpg city and 21 highway (22 highway for Silverado/Sierra), about 1 mpg better in both tests than comparable models. (For 2010, models powered by the 5.3-liter V-8 with VVT, AFE, the six-speed automatic, and standard 3.08:1 rear axle, also get 15/21 mpg, partly thanks to a new fuel-saver mode.
What’s the big deal about 1 mpg? It’s a bigger question when you consider that these more fuel-efficient specials do sacrifice some performance and capability. For example, the GM XFEs offer 7000-pound towing capacity and the F-150 SFEs boast 7500 pounds.
Because today’s EPA ratings are fairly realistic, thanks to the recent change in how they’re calculated, these trucks should be about 1 mpg better in real-world use as well. But at $2.40 a gallon at an average 18 mpg (versus 17), if you drive 12,000 miles each year, that 1-mpg difference will save $94 a year. Which raises the question: Why are these trucks just 1 mpg better? Why can’t automakers improve the fuel economy by 10, or even 12?
Fuel Economy 101
For starters, how much fuel any roadgoing vehicle consumes is first and foremost a function of its weight, or, in engineering speak, mass. Sir Isaac Newton’s Second Law of Motion says, “F=ma” (force equals mass times acceleration). It takes X energy to accelerate Y pounds to Z mph. “When you’re talking about a 5000-pound truck,” says Toyota USA senior powertrain principal engineer Dan Yerace, “that’s a lot of mass to move.” Pile on a load of cargo, hitch up a hefty trailer, and you’re looking at single-digit mileage.
Once the vehicle is up to speed, it consumes energy mostly by plowing air out of its way. Aerodynamic drag is a function of frontal area and slickness of shape -- the latter expressed as “coefficient of drag,” or Cd -- and it increases with the square of velocity. That means a truck burns more fuel at speed than a typical car simply because it’s big and blocky, and a whole bunch more at 75 mph than at 60 or 65.
Next comes powertrain, which includes engine, transmission, and drive axles. An engine’s fuel efficiency derives from its complex combination of design and technology -- much more than displacement, number of cylinders, or peak power capability. A large engine loafing typically does better than a smaller one overworking to move the same mass at the same speed. More transmission gears (a six-speed versus a four-speed automatic) offer better efficiency by spreading out the ratios -- lower on the bottom, taller on top, more closely spaced in between -- while taller (numerically lower) drive axles trade off some low-end performance for high-end efficiency. Four-wheel drive burns more fuel than 2WD, even when it’s not engaged, because it adds mass and friction and consumes still more energy when it’s being used.
After that come lesser contributors such as tire rolling resistance, friction (from bearings, seals, lubricants, brakes, and everything else that rotates and reciprocates), and electrical loads -- how much energy it takes to run the air-conditioning, the lights, even the audio system. There’s another important factor: the driver. Accelerate and brake gently, keep your tires well inflated, don’t carry more load than you need to, and you’ll burn less fuel in whatever you drive.