New Ford F350 3.31 vs 3.55 rear, interesting information

[Dale G]

I started doing some research and comparisons into someday upgrading my F150 to a new F350 diesel, SRW, CrewCab, 6 3/4' bed when this truck gets too many miles on it. I want my new truck setup to handle a bigger trailer in case we decide to upgrade that in the future. One of the "BIG" topics I have found is the old discussion of rear end gear ratios. I have found out some information that I thought I would pass along to anyone else thinking of this so it maybe saves you from having to do a lot of research like I have done. First, the Ford F250/350 SRW diesel comes with either a 3.31 or 3.55 rear end and is dictated by which size tires are on the truck. You don't get a choice of the 3.73, 4.10, or 4.30 rear end unless you have the gas engine or the DRW. So, for this setup, we are limited to the 3.31 and 3.55 rear end. If you order an option that upgrades the wheels to 20" then the 3.55 is standard. If you get the 18" wheels the 3.31 is standard. I did not mention the 17" wheels because they are not available in the trim levels I am looking at.

I prefer the Lariat package which comes with the 18" wheels and the 3.31 rear end but you can upgrade the rear end to the 3.55 and keep the 18" wheels by getting the Heavy Service Front Suspension Package. Or you can upgrade the wheels to 20" for a lot more money. The big debate I have read in numerous forums and threads is whether or not you should upgrade the rear end to 3.55 gears with the 18" tires for better towing capabilities. This is a true statement, changing your rear end from 3.31 gears to 3.55 gears will definitely give you better towing but I kept wondering why Ford would put the 3.31 gears on a F350 with a diesel that is designed to pull heavy loads. Then there are the numerous threads on the internet where people that have the diesel and 3.31 gears say they have no problems towing big heavy trailers through the mountains of Colorado.

It all comes down to final drive ratios. When you look at a vehicle's rear end ratio the size of the tires has a big effect on the final drive ratio so the size of your tires should always be part of your rear end ratio decision. I found a table on the internet that deals with tire diameter, rear end ratios, and engine RPM's at a cruise speed of 65MPH. Now this chart is for a manual transmission with a final drive of 1 to 1, but the comparison would be similar to an automatic with overdrive, the RPM numbers would just be a little different. If I remember the Ford 6 speed has a sixth gear of .64 so the actual RPM's would be lower for the automatic.

Ford uses a LT275/65Rx18E which has a diameter of 32.1" and a LT275/65Rx20E which has a diameter of 34.1". With an 18" wheel and the stock 3.31 gears the truck engine would be 2259RPM, with the optional 3.55 gears it would be at 2423RPM. With the 20" wheels and the stock 3.55 gears the engine would be at 2280RPM, and if you could get the 3.73 gears in this configuration it would be at 2396RPM. So according to this chart the final drive ratio of a 18" wheel with 3.31 gears is almost identical to the 20" wheels with 3.55 gears. These two different setups would be almost identical on their towing abilities and should get very close to the same MPG. If you put the 3.55 gears on the 18" wheels, they would be steeper than the 20" wheels with the 3.73 gears. This is why the guys that have the 3.31 gears and 18" wheels say they have no problems towing heavy loads. It is also probably the reason why Ford offers the 3.31 rear end with their 18" wheels.

I just thought this was interesting and thought I would pass the information on to anyone else that might be wondering the same thing. Sorry for the long post.

Dale

[CoChris]

Ford also offers LT275/70R18 LRE tires as the "All terrain" tires on the SRW Super Duties. Slightly taller and a little higher load rating than the LT275/65R18 tires. In the Ford 6R140 transmission (standard with the diesel), the top two gears (5th and 6th) are over drive. I have a 2017 F350, Diesel with 18" LT275/70R18 tires and 3:55 Electric locking rear end. At 65 MPH in 6th gear my engine is turning 1600 RPM. The torque peak on this year was flat between 1600 and 1800 RPM, so I am in the sweet spot at 65 MPH. When I am towing in the Colorado mountains ( with my 310GK I am typically at 22k~23k pound GCW), I typically lock out 6th gear (very easy to do with the Ford). I have NO problems with pulling power.

Chris

PS Check the tires "revs per mile" and use that rather then "diameter". It will account for load deflection and gives a lower effective diameter (about 1" less diameter), closer to actual value. I think your RPMs listed above are a little high.

[Dale G]

Ford also offers LT275/70R18 LRE tires as the "All terrain" tires on the SRW Super Duties. Slightly taller and a little higher load rating than the LT275/65R18 tires. In the Ford 6R140 transmission (standard with the diesel), the top two gears (5th and 6th) are over drive. I have a 2017 F350, Diesel with 18" LT275/70R18 tires and 3:55 Electric locking rear end. At 65 MPH in 6th gear my engine is turning 1600 RPM. The torque peak on this year was flat between 1600 and 1800 RPM, so I am in the sweet spot at 65 MPH. When I am towing in the Colorado mountains ( with my 310GK I am typically at 22k~23k pound GCW), I typically lock out 6th gear (very easy to do with the Ford). I have NO problems with pulling power.

Chris

Yeah, those tires are 33.2" tall so the 3.55 would be a better choice because they are closer to the height of the stock 20" tires. I knew that the RPM would be definitely lower with the automatic with overdrive compared to the straight manual transmission but the chart gives a good comparison. I wish I could have found a table that was closer to the .64 gears but I couldn't find one. I lock out 6th gear on my 2015 F150 ecoboost if I am pulling in hilly terrain to keep it from shifting up and down all the time. It actually seems to get a little better mileage because the RPMs are higher but the turbo boost is lower.

If anyone has the diesel with 275/65/18 tires and the 3.31 gears what is your RPM at 65MPH?

Thanks for the information,
Dale

[CoChris]

Goodyear 275/65R18 All Terain Adventure w/Kevlar LRE from Tire Rack spec page Diameter of 32.1", Rev per mile 651 (effective diameter 30.98"). With Ford 6th gear ratio of 0.67:1 and 3:55:1 rear end ratio, at 65 MPH engine will be turning at 1677 RPM.

Same conditions with a 3:31 rear end will be 1564 RPM

Chris

[Dale G]

Goodyear 275/65R18 All Terain Adventure w/Kevlar LRE from Tire Rack spec page Diameter of 32.1", Rev per mile 651 (effective diameter 30.98"). With Ford 6th gear ratio of 0.67:1 and 3:55:1 rear end ratio, at 65 MPH engine will be turning at 1677 RPM.

Same conditions with a 3:31 rear end will be 1564 RPM

Chris

Chris,
Thanks for the information. I have not been able to find any post where the owners that have the diesel and the 3.31 gears regret the choice. Same way with the 3.55, everyone seems to like that choice as well. I still haven't decided which way I will go but I have plenty of time to decide because I have no intention to get rid of this truck until it has a lot more miles on it. I am actually very satisfied with its capabilities it is just a little light and gets blown around when I am towing my camper. I figured the heavier F350 would eliminate that irritating habit.

Dale

[Jerryr]

My 2017 F350 CCSB diesel had 3.55 w/18” tires and HD front suspension.
My 2019 F350 CCSB diesel has 3.31 w/18” tires and Standard front suspension. I ordered it that way because I wanted better ride without the HD front springs.

I tow a Reflection 337RLS. I don’t see any difference in towing ability.

RPM @ 65 MPH, is about 1,500 RPM w/3.31 and 1,620 RPM with the 3.55.
Peak torque with the 6.7L diesel is around 1600 RPM so you don’t need the higher RPM to tow as you would with a gas engine.

My normal towing speed is between 62-65 MPH. At that speed, in 6th gear I have almost 900 ft/lbs of available torque.


Screen snapshot below of my 2019 w/3.31 @ 65 MPH 1500 RPM





Below is shot of my 2017 F350 w/3.55 @ 65 MPH 1600 RPM

[CoChris]

Glad to help. I chose the 3:55 rear end based on having the RPM in the torque peak at 65 MPH. I figured this gave me the best compromise between mileage and pulling power. I have reasonable (ok "stupid") power for towing and still get good mileage when empty. We use the TV for sightseeing after towing so both are important to us. The automatic covers up a lot of "gaps" in performance (i.e. you can go with a much taller rear end (lower numerical gear ratio) with an automatic than with a manual). And RAM is the only manufacture of 3/4 ton and heavier pickups to off a manual so ....

I really enjoy or truck.

Chris

[CoChris]

My 2017 F350 CCSB diesel had 3.55 w/18” tires and HD front suspension.
My 2019 F350 CCSB diesel has 3.31 w/18” tires and Standard front suspension. I ordered it that way because I wanted better ride without the HD front springs.

A little off topic, I apologize.

Jerry, do you notice any difference in ride quality? I have a 2017 F350 4x4 with the snowplow prep (heavy front springs), long bed crew cab (verrrrry long wheel base) and diesel. I find rear tire air pressure makes a HUGE difference in ride quality. I set my fronts to 65 PSI for everything. Towing I set my rears to 80 PSI. Empty, I run 60 PSI and find the ride is very good.

Chris

[Dale G]

Glad to help. I chose the 3:55 rear end based on having the RPM in the torque peak at 65 MPH. I figured this gave me the best compromise between mileage and pulling power. I have reasonable (ok "stupid") power for towing and still get good mileage when empty. We use the TV for sightseeing after towing so both are important to us. The automatic covers up a lot of "gaps" in performance (i.e. you can go with a much taller rear end (lower numerical gear ratio) with an automatic than with a manual). And RAM is the only manufacture of 3/4 ton and heavier pickups to off a manual so ....

I really enjoy or truck.

Chris

We use our truck for sight seeing as well. I probably put more miles on the truck after we get to the campsite than I do towing the camper there and back. We like getting out and seeing the area. So I am like you where unloaded mileage is important just like the towing ability. That is why I have the 3.55 in my F150. I get 20 to 22 MPG on the highway unloaded at 70MPH but it has plenty of pulling power.
Thanks again,
Dale

[Jerryr]

Let me add to my posting above.

My 2017 had some wondering at Hwy speeds and stiff steering. At Hwy speeds I could move the steering wheel 1” before the truck would change tracking. At 1500 miles I changed the A/T Goodyear Wranglers out for Michelin Defender A/S tires. There was significant improvement.

My 2019 with standard front springs, adaptive steering and Goodyear Wranglers did not exhibit any of these tendencies. No dead spots, no wandering etc.

Also with my 2017, switching to Valvoline Premium Blue Extreme 5w-40 Synthetic oil, my MPG immediately jumped up 1-2 MPG at Hwy speeds. I don’t know if was the oil, tires or engine breaking in that made the difference. For now I’m still running a Motorcraft Dino oil in my 2019. At 10,000 miles I’ll go to either the Valvoline PBE or Rotella T6.