Job 3 2008 Ford Super Dutys Get Fuel Economy Improvements
By: Mike Levine Posted: 01-02-08 11:30 PT
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Ford continues to refine the 2008 F-Series Super Duty. Only a few months after Job 2 trucks started rolling off the assembly line, Job 3 pickups are ready to go with changes aimed at improving fuel economy, without compromising capability.

Ford is replacing standard 3.73 ratio rear axles on certain F-250 and F-350 Super Dutys with 3.55 running gear and has lengthened the underbody chin spoiler by 100-mm (3.93-in).

Ford trucks spokesperson Wes Sherwood says, "We are making the 3.55 standard and when that's combined with new front-end aerodynamic improvements, there will be about a 1.5 miles-per-gallon improvement."

It's difficult to quantify the fuel economy improvement in percentage terms because three-quarter and one-ton heavy duty pickups aren't required to carry EPA mileage ratings. But this could mean as much as a 10 to 15-percent improvement for some drivers, depending on how they use their truck.

The bonus for those who frequently tow: Mr. Sherwood says there will be no decrease in capability. Super Dutys with 3.55 rear axles will still tow the same as earlier Job 1 and Job 2 3.73 equipped trucks.

For regular cab 5.4-liter Triton V8 gasoline F-250 and F-350 single rear wheel trucks, max towing remains 9,100-lbs and for regular cab 6.4-liter Power Stroke V8 diesel F-250 and F-350 single rear wheel trucks, max towing remains 12,500-lbs.

To understand why this is important, it helps to understand how rear axles are rated. Rear axles are assigned numbers that describe how many rotations the driveshaft must make to turn the rear axle (and rear wheels) once. A 3.55 rated rear axle turns the wheels once for every 3.55 driveshaft rotations.

The rear axle ratio can make a big difference in towing performance. The higher the ratio, the faster the driveshaft turns and the sooner the driveshaft can transfer peak horsepower and torque from the engine to the rear wheels. The result, generally, is faster acceleration and higher towing capacity than a rear axle with a lower ratio. The tradeoff for a higher axle ratio, though, is usually lower fuel economy, because an engine that's working harder is also burning fuel faster.

But in the case of the newly standard 3.55 rear axle, Ford isn't compromising on towing performance while it gives drivers higher fuel economy numbers. However, it's likely that 0 to 60 performance, particularly while towing, will drop with the move to the lower rear axle.

Here's a summary of Job 3 rear axle changes across the Super Duty lineup:

  • Single rear wheel (SRW) F-250 and SRW F-350 Super Dutys with 5.4-liter Triton gas V8 engines receive standard 3.73 ratio non-limited slip differential rear axles that replace previous 4.10 ratio non-limited slip diff rear axles. Expected fuel economy improvement of 0.5-mpg.

  • SRW F-250 and SRW and dual rear wheel (DRW) F-350 Super Dutys with 6.8-liter Triton gas V10 engines receive standard 4.10 ratio limited slip rear diff axles that replace previous 4.10 ratio non-limited slip diff rear axles. No change in fuel economy.

  • SRW F-250 and SRW F-350 Super Dutys with standard 17-inch and 18-inch wheels and automatic 6.4-liter Power Stroke diesel V8 engines receive standard 3.55 ratio non-limited slip rear diff axles that replace previous 3.73 ratio non-limited slip diff rear axles. Expected fuel economy improvement of 1.5-mpg.

  • SRW F-250 and SRW F-350 Super Dutys with optional 20-inch wheels and manual or automatic 6.4-liter Power Stroke diesel V8 engines receive standard 3.73 ratio limited slip diff rear axles that replace previous 3.73 ratio non-limited slip diff rear axles. No expected fuel economy improvement.

  • DRW F-350 Super Dutys with 6.4-liter Power Stroke diesel V8 engines receive standard 3.73 ratio non-limited slip diff rear axles that replace previous 4.10 ratio non-limited slip diff rear axles. No expected fuel economy improvement.

Asked why these changes are being made at this time, Mr. Sherwood says, "Ford is constantly looking to innovate with its trucks. Fuel economy is on people's minds right now and we're not going to wait until the next model year to make a change. We're going to implement changes like this as soon as we can."

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