I'm a retired freight trucker so I know how important it is to match torque with ax ratio and the torque curve with desire speed. A lot good comments made here so could someone post the 6.7 torque curve that would show us the best ax ratio towing 16000 lbs 5th wheel camper at 65 mph. I'm old school so I think lower rpm means better mpg??? I looking at getting 17 F350 4x4 CC, SW, SB King Ranch. I don't know what ax ratio to get. I travel 95% flat ground...

 

3.55 is about perfect in my mind for a SRW and your weight. My last rig was 16k and with my 400/800 motor, it was just about right. Maybe wanting for a little. With the new ratings, I think you would be in the perfect sweet spot.

 

We actually have that discussion going over in the Numbers thread. I think the 3.55 is ideal although others seem to seek the higher RPM of the higher ratios. I will post the 2016 torque curve below which reaches peak at 1600. 2017 reaches peak at 1800 according to the one graph I have seen. Also keep in mind these transmissions can be operated in M manual with paddle shifters so you can better keep your preferred torque range.


10 years OTR and 3 years dispatch and terminal manager.

 

I figure I want the maximum power for climbing hills. The 3.55 on the 20" tires gives you ~2600 RPMs in 4th gear at 65MPH. You hit peak HP around 70 MPH - perfect. It also runs 1525 RPMs at 65 in 6th, which is great for mileage.

The 3.31 is 2440 RPM in 4th at 65 and 2630 in 4th at 70. 1420 RPMs in 6th at 65 MPH - more downshifting and similar mileage. There is significantly less torque and even less HP at 1420 RPMs than 1525 RPMS.

It is noteworthy that the F450 with its 4.30 gears and shorter tires runs nearly the same RPMs as the 3.55, but one gear higher. 65MPH in 6th = 1973 RPMs and in 5th = 2533 RPMs. The F450 hits max speed in 4th around 55 MPH, so pulling hills on the highway is done in 5th or 6th.

 

3.55 for sure. Never saw a noticeable difference in fuel economy with the 3.31 trucks. Nor did the 3.31 trucks feel sluggish at all, but the 3.55 feels slightly more responsive. In reality, it's not a big enough difference in gearing to make a big difference in performance or fuel economy.

 

The 3.31 ratio is not available with either the camper package or the snow plow prep package. There is a tremendous amount of torque available and although not by much the higher ratio would be somewhat less stressful on the driveline, especially in close quarter maneuvering.

 

I had 3.31 and 20's in my 13 SD but didn't much care for the combination as it felt a little sluggish to me. My 17 will have 3.55 and 18's. If I towed heavy all of the time I would want 3.73's but that's not available on a SRW

 

I personally went with the 3.31's and 18's on my order. I had 35 inch tires on my otherwise stock 2004 6.0. Power was not the limiting factor in what that truck could tow, and it "only" had 560 lb-ft of torque. With 925 lb-ft on tap, I can't imagine that 3.31 gears will be much of a disadvantage.

 

I also tend to agree the 3.55 ratio is the best overall. I have it in my current 2016 F-350 Crew 4x4 SRW SB truck with 440 / 860. Unloaded, it will deliver pretty near 20mpg at 70mph on the interstate. At 55mph, unloaded, it will deliver 25 mpg. Loaded, with 10k behind it, it will still return 13mpg.

My ordered truck is a DRW 350 and there were only two options: 3.55 or 4.10. Mine is 4.10. I'm somewhat regretting that now since I do drive unloaded a fair amount as well. 3.55 seemed awful low for a dually at the time of the order, but with the extra torque it would probably have been fine. There is no way I'll ever see 20mpg at 70 mph unloaded. Probably more like 16. City and towing might possibly improve, however.

 

3.55 for sure. It's too bad a 3.73 isn't an option, that gear set would work well for the larger tire guys that tow a lot.

 

Interesting that 3.73 isn't available anymore. What ever happened to it>

 

Would have been exactly what I wanted to order...not available.

 

I suggest that 3.73 would be inferior to the 3.55 for towing on the highways - at least the ones that are 70 MPH. With the 3.55 and 20" tires, you are right at peak HP at 70 MPH (2800 RPM) in 4th gear for the hill climbs. The 3.73 would only be ~150 RPM higher, but that produces less power and runs the engine harder.


The problem with the 3.31 is that you are well below the 1600 torque peak until you exceed 70 MPH. When towing at, say 55 MPH, you will be downshifting a lot more - not because of the slightly lower multiple on the torque, but because the engine power drops off dramatically below 1400 RPM. 3.31 runs 1200 RPM at 55 MPH compared to just below 1300 RPM for the 3.55. The 4.10 (F350) / 4.30 (F450) dually gears especially shine there.

 

3.31 for me. I will see how this works for me. been with 3.73s in my 99

 

With the torque rise at 1800 rpms this year it will be interesting to plot the performance vs gear vs speed. I ordered 3.55 but would have chosen 3.31 if the computer would allow with the camper package. Now with the increased torque peak I am glad for the 3.55.
As far as downshifting I am pretty sure the 10 speed will not come into the world of the 6.7. It is un-necessary with the torque range but I guess I understand it for the F-150 and the need to provide more and better options for the advertising, and maybe the EPA.