I am looking at a 2017 F350 SRW/Diesel and the only option seems to be 3.55 or 3.31. Does anyone know why there is no 3.73. You have a choice between 3.55 non slip or 3.31/3.55 electronic but that is it.

 

3.55 is a good ratio. I was in the same camp as you regarding gearing but with the new transmissions you won't miss the 3.73 one bit.

 

It was that way on the 2016 Super Duty as well.

 

3:55s are plenty unless you start adding tires larger than 35s and expect it to tow decent.

 

I was hoping for at least 3.73's, on my '05 I swapped out the 3.73's with 4.10's, it was ok but I should have gone to 4.30's.
I'll see how it goes with the 6.7.

 

You must also keep in mind you have a far superior transmission to the older units. The high final drive ratio isn't needed like it once was for startability.

 

As TQ ratings continue to increase, steeper gearsets become less necessary. As stated above, 3.31 and 3.55 have been the only two SRW 6.7 options since it's inception in 2011. Now that the TQ has risen again for 2017, even the DRW 350's are coming with 3.55's instead of 3.73's with 4.10 as an option as opposed to the previous 4.30 option.

 

I'm use to my 4.30 in my 2015 F450. I was startled by the 3.55 in a F350.

On a side note very little price difference between a SRW F350 and a F450. A loaded SRW 350 is nearly $80,000 while a F450 is around $83,500.

 

The 3.55 is nearly the perfect gear for the 34" tires on that truck. You will run right at peak HP at 70 MPH in 4th gear. With a 3.73, you hit peak HP around 65 MPH. Not a really big difference, but I prefer to have maximum hill climbing power at 70. Shifting to 5th gear WOT right at 65 is not good in my book. It drops you down to 2100 RPM, which is way less power... slow down, shift back to 4th, speed up, shift - slow down...

On a related note, the F450 hits peak HP at 72 MPH in 5th - also a great gear combo.

F350 DRW 3.55 peak HP = 65 MPH - shifts to 5th WOT at 65 - suboptimal for me
F350 DRW 4.10 peak HP = 75 MPH - way better than the 3.55 DRW for me, but not as good as the 4.30

 

Along with the lower ratios and increased torque comes the need for stronger and more durable differentials. Thus the change to the M275 (SRW) and M300 (DRW) Dana axles. And at that the F450 has a stronger larger diameter axle shaft as well. The M300 in the F350 has a major shaft diameter of 1.47" while the F450 has a major shaft diameter of 1.85".

 

I believe the 3.55/3.31 ratios predate the 6.7 and started off sometime during the 6.4. 3.55 was also an available ratio on the original 7.3's.


The 6 speed transmissions have lower first gears than previous transmissions. You've got a lower starting ratio even with higher axle gears. You can lock out sixth/double over or fifth/single over to get your rpm's where you need them at cruising speed while towing. The higher axle gears get you to a better empty cruising rpm for mileage.


The only thing lower axle gears would accomplish would be to get you into higher rpm's at highway speeds. That could give you all six gears to tow with and keep your splits closer, but I have a hard time seeing that as worthwhile overall for most people.

 

The 3:55s started with the 6.4.

 

SD is a torque monster, don't have to worry about 3:55's.