What follows is a series of tests a buddy and I did using his Silverback by Cedar Creek 5th Wheel RV. It's 40 feet long, and loaded as it was, it weighed in around 14,000#.
We did mpg tests, suspension squat tests, and 0-60 runs with each truck.
My truck is a 2014 Ram 2500 Crew cab, 4x4, 6.4L Hemi, 66RFE auto, 4.10 axles, Laramie pkg, equipped with the Ram rear coil springs without the air suspension. The 6.4L Hemi is 410hp/429tq.
His truck is a 2014 Ram 3500 SRW Crew cab, 4x4, 6.7L Cummins, 68RFE auto, 3.42 axles, Laramie pkg, equipped with the Ram 3500 rear leaf springs an no air suspension option. His engine is the 370hp/800tq version.
Both trucks have the same wheels and tires, and are completely stock with under 5000 miles on each truck. I drove my Hemi truck for all tests, and my friend (SKG) drove his Cummins truck for all tests.
As far as the squat testing goes, we spent a little more time and took good measurements. The RV is a 40' Silverback by Cedar Creek 5er with 4 slides. It has a 15,500# GVWR and likely ~2700 to 3000# hitch weight.
We measured each in the same spot on the street in front of SKG's house hitched and unhitched, and the space before the bump stop would make contact on each truck.
RAM 2500 6.4L Hemi (with rear coils)
Total sag, 2.7"
Space between jounce bumper and axle perch (before suspension bottoms out) when RV is hitched up, 1.8"
Overall view** Keep in mind, my 5er hitch is set too high for his RV and the airbag pin gizmo he has on there, so the RV was a bit nose high when hitched to the Silver 2500 Hemi (my truck, SKG's RV).
2014 RAM 3500 SRW (rear leaf springs)
Total sag, ~2"
Space between jounce bumper and axle tube (before suspension bottoms out) when RV is hitched up, 2.75" (angle is bad for camera, but it was just under 3" by eye)
Overall view at same spot on highway:
That's all the static testing covered. Now onto the driving tests.
0-60 test towing 14,000#
Impressions (my opinion):
The Hemi built power smoothly and had the more power at higher RPM (duh). It was a very predictable flow of power, and didn't seem "lacking" at all. It does have the characteristic flat spot in the power as soon as it changes from 1st to 2nd gear as the step between those gears is pretty large. It takes the engine a few seconds to recover from that gear change. I would best describe it as "adequate" power (IMO) for the load. The "clunk" you hear right at the end is from the fancy airbag hitch on the 5er.
Hemi 0-60 towing 14,000#
The Cummins powered truck was a completely different experience. It tugged like crazy in first gear from about 1500 - 2500 RPM, then the power kind of drops off at higher RPM and as the trans shifts between gears. Fully uncorked and not held back by the computer, the power is much more evident than with the Hemi. I don't care what anyone says, the power output is definitely being "managed" by the computers. There are stark differences between full ON (I'd call "impressive") and tq managed when the accelerator is held to the floor. When the RPM and speed are right and the computer isn't choking the power it pulls amazingly hard - even with the heavy load we had behind it.
Cummins 0-60 towing 14,000#
MPG test towing 14,000#
OK, firstly, I know this is not as scientific as could be done. But remember, we're just a couple of guys, not scientists. We ran each truck at the same speed on the same road, towing the same RV one after another. We let each "settle", after accelerating from a stop, then started the video recording. The passenger did the recording, so the angle isn't ideal, but you can see the mpg display on each hopefully.
I also didn't want to post a 10 min video of each test. It takes FOREVER to upload and it would be boring to watch. Each video is around a minute long.
We each had a side/corner wind to deal with, but it had a much more detrimental effect on the Hemi mpg as it had to work harder to maintain speed. The Hemi would run in 4th gear at about 2500 RPM (6 speed auto trans) when hitched up. With my lighter 5er (it's about 12k#), it will usually run in 5th. The Cummins quickly settled into top gear (6th) at just under 1400 RPM, even with the load.
6.4L Hemi & 4.10 gears - mpg towing 14,000#
6.7L Cummins & 3.42 gears - mpg towing 14,000#
The side wind we dealt with gave us a chance to compare the chassis and road feel of each truck. The 5er we were towing has a very large, high, flat front cap, and a tall side profile. Personally, I didn't feel either truck was "unstable" at all. Each felt confident and stable; it was quiet and relaxed in the cab. I will say that the 3500 SRW felt a little "stiffer" in the rear than the 2500 - makes sense I guess based on the squat measurements. If the 2500 was a 9/10 for stability (just an example), the 3500 SRW was a 9.5/10.
I welcome your comments.