This article originally published in Issue 59 of the Turbo Diesel Register.

TDR REVIEW

DRIVING THE 2007.5 6.7L AND 68RFE

by Jim Anderson, TDR Writer

There it sat, hunkered down in the Turbo Diesel Register’s Georgia parking lot. The new TDR company ride is a 2007.5 Mega Cab 3500 dually long bed, dressed in silver paint with black wheels, which makes it sort of sinister and serious looking. The front view is changed by the addition of the hood from an SRT 10 model, which makes the truck look like a shark on the prowl for prey. Because of its looks and size, this truck is altogether a bit intimidating. Editor Robert Patton has also added an E-Z Ride rear suspension which lowers the rear of the truck by 1.5 inches and provides rubber to cushion road bumps rather than depending solely on steel springs. Further emphasizing the long, low appearance of this truck is the long pickup truck box. Yep, you read it right; this is a conversion truck, with 22 inches added to the frame length (see page 118).

The reason for the gathering is to check out the changes and form driving impressions between this 2007.5 truck and my 2006 truck. These two trucks share similarities in that cab styling and front end sheetmetal are the same, and both are two wheel drive 3500 series trucks with eight foot pickup beds, dual rear wheels, and automatic transmissions. There are also many differences such as engines and transmissions, so this article is not intended as a head-to-head performance comparison.

At first glance the most obvious difference is truck length, as the 2007.5 Mega Cab Long Box conversion is about two feet longer. The cab’s rear seat is so large it reminds me of the rear seat in an early ‘50s Nash Ambassador. I’ve seen tighter rear seats in limos and Checker taxis. Although both trucks were purchased as haulers and for towing, each of our needs is different. Robert needs to haul stuff in the bed of his truck while also towing a trailer and he also has the need for lots of cab room for hauling people. I don’t often haul people, prefer a fifth-wheel trailer, and need a shorter truck for getting into tight campsites.

Here are the specs of the two trucks:

2006 Dodge Quad Cab; SLT package; 3500 dually; 5.9-liter Cummins engine; four-speed 48 RE automatic transmission; two wheel drive; 4.10:1 axle ratio. Scale weight: 8244 pounds.

Main accessories: Pac Brake exhaust brake; spray in bedliner; 24K fifth-wheel trailer hitch; 70 gallon auxiliary fuel tank; tool box in bed; fixed tubular cab entry assist steps, boost, EGT, and trans temp gauges.

Approximate fuel mileage: 19-21 solo, 10-12 towing.

2007.5 Dodge Mega Cab; SLT package wheel drive; 3.73:1 axle ratio. Scale weight: ??

Main accessories: Integral exhaust brake; spray in bedliner; bed cover; load equalizing receiver hitch; AMP retracting cab steps, boost, EGT, and trans temp gauges.

Approximate fuel mileage: 13-17 solo, 10 towing.

The 2007.5 engine has a larger displacement at 6.7-liters, though it and the earlier 2006 5.9-liter engine are approximately the same physical size. Because it is based on the older 5.9-liter design, the internals and design are both well proven. Both are inline six cylinder engines with Bosch high pressure common rail injection systems. Both are undersquare meaning the piston stroke is longer than the cylinder bore is large, the same design as used on virtually all large truck inline six diesel engines. The design yields more torque at lower engine speeds than competing short stroke V-8 engine designs. There is more torque available off-idle speed and the torque band is broader. The new engine already meets the next level of exhaust emissions slated for 2010 while boasting an increase in peak torque to 650 ft-lbs. Horsepower is now rated at 350 on automatic trucks.

The new 68RFE transmissions installed behind the 6.7-liter diesel engine is still based on the old Chrysler 727 automatic transmission that has been around since the 1960s. The new transmission is a six speed, interposing a gear between second and third gear and another between direct and overdrive. The added ratios give more versatility in gear selection to keep the engine in its optimum power range more of the time.

With the mid-year (2007.5) introduction of the 6.7-liter engine the automatic transmission was revised to a Chrysler-supplied 68RFE, six-speed unit.

The computer programming for the transmission has also been greatly revised and improved, now allowing full gear selection and engaging the torque converter clutch as needed in the top five gears. It is fully integrated with the turbocharger exhaust brake, too.

To cut to the chase, the 2007.5 truck is altogether a better truck than the 2006 it is compared to, with one exception that I’ll cover later in this article.

My first impressions of this 2007.5 truck when you get in and start it up is that the 6.7-liter engine sound is quieter and with a deeper note, which is to be expected of the larger engine. The transmission shift pattern on the dash display shows only “P, R, N, and D.” On the shift lever is the expected tow/haul button and a handy +/- rocker switch to provide control of top gear selected in the transmission, which also adds a number to the dash display. In the lower center of the dashboard is a switch that enables exhaust brake activation. Otherwise, the dash layout is the same as the 2006 truck’s dash, with white face gauges. There is one other difference in that where the 2006 truck’s SLT interior came with fake wood accents, the 2007 truck has a shiny brushed aluminum accent, which editor Patton has repainted black. I like the black better, too, as the aluminum color plastic looks sort of chintzy.

I hop into the ’07.5 Mega Cab and I grab reverse gear and back out of a tight parking place, paying particular attention to the fact that this truck is nearly as long as your average Navy aircraft carrier. That back end looks awfully far away in the rear view mirrors. You’ll find yourself picking shopping center parking spaces farther from the door, since this truck takes up a lot more room than a single space offers. I am soon in forward gear and on my way down the street. The new 68RFE automatic transmission is completely unobtrusive as it goes about shuffling through six gears, locking and unlocking the torque converter clutch as required. It is difficult to detect gear changes, much less keep count of how many gears this transmission has shifted through. During the whole process of coming up to highway speed, the engine provides plenty of low-end torque and seldom turns above 1700 rpm. Light throttle operation will find the transmission upshifting at 1300 rpm. Rolling down the road on level ground, the engine may turn as slowly as 1100 rpm in top gear, which is a double overdrive. Under most conditions, the engine pulls strongly from low engine speeds without a downshift unless you really stick your foot into the fuel pedal. Yep, both fifth gear and sixth gear are overdrive gears to provide just the right engine speed for road and load situations. The truck has a much less choppy ride than the 2006 truck, largely due to the aftermarket rubber rear suspension and this truck’s longer wheelbase. The unloaded 2006 truck’s ride is “nervous” over small bumps, but smoothes out when loaded. If you drive across a frost heave or railroad tracks with either truck you’ll still get a slap in the back as the four rear tires get kicked up by the bump. There’s a lot of unsprung weight moving around back there. The 2007.5 truck feels a bit less nimble than the 2006 truck, but it handles well and goes where you point it. You soon get used to the longer length and quit worrying about catching a curb on right hand corner turns.
How about noise? You’ll see editor Patton’s chart (page 44) comparing the noise level of the two trucks. These numbers show a readily discernable difference, so Dodge and Cummins engineers have continued to make improvements in the area of noise, vibration, and harshness.

Switching to the tow/haul mode and turning on the exhaust brake invokes a different transmission computer program wherein the transmission engages each lower gear as you slow down and enables the exhaust brake for each new gear selected. You don’t have to play with the shift lever and the gas pedal to get this effect as you do on the 2006 truck. The integral (and supplied as standard equipment) exhaust brake on the 6.7-liter engine is fully as effective as the aftermarket one on my 2006 truck. At heavy throttle, the 2007.5 truck’s shifts are still smooth, not as grabby as on the 2006 truck. And for such a heavy truck, it moves out with authority. It has enough torque right off of idle to spin the tires starting off any time you desire to put down some rubber, and the power is solid to the red line with no flat spots in acceleration. There’s absolutely no exhaust smoke, either, due to the more stringent emission controls including EGR, catalytic converter, DPF, and other doodads that make it run cleaner than the 2006 truck. This engine also runs hotter, and the EGT gauge can show up to 1200°, even at light throttle as the engine computer feeds diesel fuel through the engine to provide the heat used to burn off accumulated carbon in the particulate filter in the exhaust pipe.

We hooked a 12,000 pound, 30-foot long cargo trailer to the 2007.5 truck to test towing operation. Because this truck has been stretched to accommodate the long bed and therefore weighs more, I suspect this trailer/truck combination is at the maximum combined weight rating. Again, the engine has more than enough pull to easily handle this added load, and the transmission is so versatile that you’re covered in any towing condition with the right gear selection every time. First gear is a lower ratio (3.23) to aid starting a load from rest, even uphill, without any strain. Though I tried, I was unable to confuse the transmission computer. For towing, I mashed the tow/ haul button and enabled the exhaust brake button. By pushing the + or – button on the shifter you can easily select any gear manually, or just sit back and let the truck do its thing automatically. In short, this transmission is a delight. There’s simply no reason to buy this truck with a six-speed manual transmission, as the automatic offers just as much versatility and control in any towing situation, and that’s a strong recommendation from this ‘ole man who has been forced to change his mind about automatics. We’ll soon be able to answer the only remaining question about long term durability as members accumulate miles on their trucks.

In towing situations at high speed on highways, the truck and its load tracked well and the engine was quiet, with plenty of power in top gear to maintain speed on most hills without a downshift. The engine and transmission programming prefers to rely on the engine’s torque to maintain or gain speed rather than making a downshift, unless you press pretty hard on the throttle. And that’s as it should be with these trucks. On steeper two lane roads at slower speeds, the exhaust brake automatically went to work downhill to retard speed, and on the climbs, the transmission downshifted as needed to keep the engine producing enough power to maintain speed, but didn’t drop two or more gears to cause the engine to wind itself to the red line as my 2006 truck does. In city traffic, very little throttle was needed to get the load moving, and you could hold the transmission in 1,2, or 3 for stop-and-go situations. Because reverse gear is lower (higher numerically), it was easy to slowly back this long rig into tight spots, and there was almost no driveline shuddering while backing, unlike my 2006 truck.

When you turn off the ignition key, the transmission programming and exhaust brake switches default to the normal settings, so when you restart the truck, these switches must be turned on again if you want the exhaust brake enabled and/or the tow/haul mode enabled. So far, members haven’t discovered a way to overcome this switching arrangement, though I’m sure it is just a matter of computer programming. Some members have complained to me about this as being a problem for them.

When the exhaust brake is enabled, it will not stay on at idle speed as aftermarket units do. But during periods of extended idling, the computer turns it on and off occasionally to keep combustion chamber temperatures high enough to work with the exhaust aftertreatment devices.

Since the turbocharger bearings are cooled by a water jacket, there is no longer a need to closely monitor EGT prior to engine shutdown. If you wait until the temperature gets down to 300° as on earlier models, you’ll likely sit there for a long time, burning diesel fuel needlessly. Idling for a minute or two after a hard drive should be enough to blow most of the exhaust heat out of this new engine, and the coolant jacket will take care of the rest.

In comparing the 2007.5 truck to my 2006 truck, I’m sorely tempted to trade if only to get rid of the vexing four-speed automatic transmission that so sorely lacks in towing versatility and control. If the bright idea occurs to you that I could simply swap transmissions, forget it, since the computer that controls it all would also have to be swapped and wouldn’t mate up with the older 5.9-liter engine. Rather than re-engineer the older truck, it would be less expensive to trade for the new truck. Again, I’m tempted.

Now to the one drawback the newest model has, and there’s not much we can do about it. This biggie is lower fuel mileage mostly due to the government mandated emission control stuff that is required of all diesel engines made after 1/1/07, no matter what brand of truck you buy. There’s likely no way around it, no way to disable or disconnect it, and no way it’ll pass an emissions check without all this stuff attached. And it will cost you up to 2-5 mpg versus a 2006 truck every time you drive the 2007.5 truck. It’ll be up to you to decide whether the higher fuel costs will outweigh your desire for a truck with an engine-transmission combination that is improved in every other way over its predecessors.

Whoops, there’s one more drawback: that of price. The 6.7­liter engine option lists at $6,100 and the six-speed automatic transmission option lists at $1,575. Both cost considerably more than on previous models, so you’d better have more money in your pocket before you go shopping. To add insult to injury, some states are now tacking on an additional number ($995 in the case of Tennessee) as a state mandated charge (or tax) for the privilege of purchasing a new vehicle. I have been unable to find out what the state(s) plan to use this money for, but I bet it won’t be to our advantage.

Editor’s Notes: At the conclusion of my test drive of both trucks, I commented, “Jim, let some of the air out of your tires. It rides a little rough.”

The rougher ride could be attributed to the air pressure, less tread on the tires, shorter wheelbase, noisier engine; you pick one. I did not care for the 4.10 ratio differential in Jim’s truck, but the good old 48RE seemed quite adequate. I did not drive Jim’s truck with any kind of trailer in-tow.

Maybe there is just a fascination for new…

Depth perception is...

Patton's Mega Cab Long Box seems to dwarf Anderson's Quad Cab,
at least until you reverse the position of the trucks.