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There are other build differences too.

Chrysler’s Jim Repp, engineering supervisor for commercial vehicles, describes, "This is the same basic architecture as the 3500 from a looking standpoint, but on the 4500/5500 trucks the frame rail is 2mm thicker - so it’s 7mm thick versus 5mm - which makes it much stronger. You'll also notice that if you compare a 3500 to a 5500 you'll see much larger spring hanger brackets, springs, and larger axles."

The 6.7-liter Cummins motor has been modified for the 4500/5500 Chassis Cabs. Sharp eyes will note that the inline six cylinder is detuned to 305-horsepower / 610 lb-feet of torque, from 350-horsepower / 650 lb-feet in the Ram 2500 and 3500 HD pickups.

"This is related to the emissions 'bins' [the EPA separates vehicles into different bins depending on GVWR and exhaust cleanliness – M.L.]. Pickup truck emissions are different from chassis cabs emissions [medium duty chassis cab emissions rules are less strict than those for light and heavy duty pickups – M.L.]. One is a full vehicle certification while the other is a dyno cert. And we also tune the 4500/5500 for the commercial market, for fuel economy and usability as well," explains Mr. Repp.

The 4500/5500 Cummins' exhaust line traps and burns off soot like the Ram 2500/3500 Heavy Duty pickups, but the diesel particulate filter is mounted underneath the cab in the Class 4/5 trucks instead of under the bed in the HD Rams. And while the 2500/3500 HD pickups meet 2010 emissions standards for Nitrogen Oxide (NOX) today, using Cummins' near maintenance-free Adsorber catalyst, the 4500/3500 trucks will rely on urea selective catalytic reduction (SCR) to scrub NOX molecules. The urea tank will be mounted on the chassis by 2010 and will require periodic refills.

Unlike the 3500 Ram Chassis Cab that offers a choice of HEMI V8 or Cummins I6, the 4500/5500 trucks only come with the Cummins mill. "That’s because of the higher GVWs," says Mr. Repp.

The standard transmission is a Mercedes designed G56 six-speed manual gearbox. The G56 replaces the old New Venture NV5600 manual six-speed and has origins in much heavier Class 6 trucks.

There’s also a new six-speed automatic, built by Aisin Seiki in Japan. The internals have been around for several years but Chrysler built a new housing so it would fit the 4500/5500 driveline. The Aisin is a different automatic than the six-speed 68RFE in the 2500/3500 pickups. It has tow/haul mode but does not offer tap up, tap down shifting.

Both transmissions are power take-off (PTO) capable, providing Chassis Cab owners with the foundation to run separate machinery off their truck’s engine. A hydraulic pump mounted on the transmission can power up to 35-horsepower PTO equipment.

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