GM trucks tweaked for efficiency

  • 04-Sep-2008 03:26 EDT
XFE Silverado.jpg
The 2009 Chevrolet Silverado XFE (shown) and the GMC Sierra XFE have increased city and highway mpg ratings. In addition, increased tow ratings from 6600 to 7000 lb (3000 to 3200 kg) are due to the switch to a six-speed automatic transmission and a high-capacity cooling package.

New full-size pickups and SUVs from General Motors sporting the XFE moniker, which stands for eXtra Fuel Economy, have better city and highway fuel-economy ratings than the automaker's non-XFE models.

The XFE versions of the 2009 Chevrolet Silverado and GMC Sierra pickups as well as the Chevrolet Tahoe and GMC Yukon SUVs carry EPA fuel economy estimates of 15 city and 21 highway, a one mpg improvement versus the 14 city and 20 highway ratings of comparable non-XFE models.

XFE models are the result of an engineering pursuit launched in early 2008 that focused on altering GM's full-size trucks and SUVs through a number of mechanical, mass reduction, and aerodynamic improvements.

Silverado and Sierra XFE pickups have a 0.412 Cd, representing the full-size truck segment's best aerodynamic number. "The soft tonneau cover is worth about 65% of the aero improvement on the XFE package," said Jeffrey Luke, Chief Engineer for GM's Full-Size Trucks. Other aero alterations include lowering the front air dam by 1 cm (0.4 in) and lowering the rear suspension by approximately 2 cm (0.8 in).

"The aero improvements are most effective on the highway test—accounting for approximately 60% of the highway fuel-economy improvement—and about 19% of the fuel economy gain on the city test. Mass savings play a greater role in the fuel-economy improvement on the city test," said Luke.

GM's XFE pickups and SUVs are powered by a 5.3-L FlexFuel V8 engine with an aluminum cylinder block and heads. The engine is rated at 315 hp (235 kW) and 338 lb·ft (458 N·m) in the Silverado and Sierra applications and 320 hp (239 kW) and 340 lb·ft (461 N·m) in the Tahoe and Yukon applications.

"The two-wheel-drive Crew Cab pickup truck used an iron block, steel lower control arms, and a steel spare wheel in 2008," said Luke. Steel or aluminum road wheels were offered in the previous model year. In addition to the aluminum engine, the XFE trucks have aluminum wheels and aluminum lower control arms.

All XFE models use a 3.08:1 automatic locking rear axle versus the 3.42:, 3.73:, and 4.10:1 axle ratios used with the 5.3-L engines in the 2008 model year. "The 3.08 axle allows the engine to run at lower revolutions where it's more efficient," said Luke. "Lower axle numeric numbers have generally meant better fuel economy but slower acceleration. The exciting thing about the XFE is that, with the new Hydra-Matic six-speed automatic transmission, the 3.08 ratio launches aggressively, like a 4.10 ratio in a four-speed, and cruises efficiently with more overdrive than a 3.42 ratio in a four-speed automatic. What you get is the best of both worlds—quick, aggressive launches and great fuel economy."

More fuel-minded technology for GM vehicles is on route. "You can expect us to keep improving fuel economy on the XFE as well as the rest of our lineup," said Luke. "We've been the fuel-economy leaders on full-size trucks for several years, and we plan to keep it that way. You can expect future models to incorporate improvements in all these key areas—powertrain efficiency, aerodynamics, mass, electrical load, and tire rolling resistance—as well as what I call the important odds and ends, such as brake drag, driveline drag, controls, and accessories."

For 2009, 18 GM models have EPA highway fuel economy of 30 mpg or better, including the Chevrolet Cobalt XFE and the Pontiac G5 XFE subcompacts. Although the technical modifications differ from the changes made to the XFE full-size trucks and SUVs, the end result is the same: higher mpg when compared to the non-XFE counterparts.

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