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Old 09-27-2015, 06:45 AM   #1
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3.31 rear end vs. 3.55 rear end

Question from the uninformed. Buying an F350 diesel, torque remains the same regardless of which rear end I purchase. I have plenty of payload capacity, haul capacity as I am not hauling a large fifth wheel. I would like some pros and cons to help me determine which rear end to get, 3.31 or 3.55. Is there much difference in the gas mileage and acceleration? Thanks in advance,
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Old 09-27-2015, 06:58 AM   #2
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Although torque at the engine remains the same, the lower gear ratio of the 3.55 will increase torque at the rear wheels (at the cost of higher RPMs for the same speed) Not sure how it will effect mileage when under a load.
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Old 09-27-2015, 07:12 AM   #3
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Engine torque is measured with the engine mounted on a dyno with no consideration given to final drive ratios. I am not a big fan of high (lower numericly ) ratios. I like the power out of the gate with the lower ratios. Myself, I would prefer something in the 3.9 to 4.1 range if I were buying this truck.
You might lose a small amount of fuel mileage but you would be very happy with the pulling power with a load on hills or a strong head wind. Under some circumstances the mileage might even increase because the engine isn't fighting the load as much requiring you to push harder on the go pedal.
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Old 09-27-2015, 08:01 AM   #4
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The 3:55 gears are a nice combo. Never know when you might want to go larger.
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Old 09-27-2015, 09:19 AM   #5
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My current F250 is my daily work truck, and has 3.55's. I wish it had 3.31's for fuel mileage. At least on paper, your not giving up anything for towing. I drive 100 - 200 mi / day, with some off-road every day. I rarely pull anything, and never more than a 16' equipment trailer. I wonder if there is any significant difference in mileage. I'm averaging 14.6 MPG combo Hwy/City/Off Road.
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Old 09-27-2015, 09:56 AM   #6
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3.31 rear end vs. 3.55 rear end

What will you be towing, how heavy is it, how often? It's all a trade off as you can't have it all. For towing the 3.55 is obviously better but for daily driving and fuel mileage when not towing the 3.31 is better. My F450 has a 4.3 rear end, excellent for towing but I use it as a daily driver also. I've gotten use to driving 65-70 vs. 70-75 because my gas mileage goes down significantly above 70 mph. Someone needs to invent an adjustable fear end. Personally, I'd get the 3.55 since I bought my truck for occasional towing though most of my driving is not towing.
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Old 09-27-2015, 10:13 AM   #7
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I think that the ground rules have changed with the newer diesels and 5/6 speed transmissions. You can have the taller rear (3.31) for economy and if need be tow your heavy trailer in 5th. I don't think that you have to run such a low rear unless you have an F450 with really tall tires. I see guys towing on the highway all the time and with many of them their diesel motors are SCREAMING.

I had an 07 Dodge with the 5.9 Cummins and with the standard rear I was running 2,000 RPMs at 65, right in the heart of the torque band. I would pull an 8,000 pound box trailer and it would almost never get out of high gear. It would just groan a little more and chug right up any hill.

Now I have a 38' motorhome with the same Cummins ISB (6.7 though) and with the Allison 6 speed trans it goes down the road at 1700 RPM at 65 in 6th gear. Of course it is running more than twice the weight of my pickup/trailer combo so when we hit a rise it downshifts to 5th, but again that is still only 2,000 at 65. On the tougher climbs it goes down to 4th, and runs around 2,500 to do 60. So based on my experiences, if I were to get another Diesel pickup, I'd get the tall rear and let the trans do the work when towing.
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Old 09-27-2015, 11:02 AM   #8
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Ford used the 3.55 gears with the 20" wheels. The 3.31 gears with the 18" wheels turned almost the same RPM's. So as you decide on what to buy, compare apples to apples. I chose the snow plow package because it gave me the 3.55 gears with the 18" wheels, I don't know if any of this changed recently.

Folks like to throw out gear ratios without looking at the whole picture. Keep in mind that the diesel transmission has 5th gear as overdrive and 6th gear as over-overdrive. You can't compare gear ratios to another truck that uses a different transmission or different tires.

I doubt that you will find much difference in fuel usage with any choice you make. I like my setup as it is better for the transmission and shifting is reduced.
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Old 09-27-2015, 11:49 AM   #9
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Quote:
Originally Posted by lynnmor View Post
Ford used the 3.55 gears with the 20" wheels. The 3.31 gears with the 18" wheels turned almost the same RPM's. So as you decide on what to buy, compare apples to apples. I chose the snow plow package because it gave me the 3.55 gears with the 18" wheels, I don't know if any of this changed recently.

Folks like to throw out gear ratios without looking at the whole picture. Keep in mind that the diesel transmission has 5th gear as overdrive and 6th gear as over-overdrive. You can't compare gear ratios to another truck that uses a different transmission or different tires.

I doubt that you will find much difference in fuel usage with any choice you make. I like my setup as it is better for the transmission and shifting is reduced.
You are correct and it makes sense as to why the tow rating is the same. The Build Your Own site will not let you select 20" wheels with the 3.31's. It tries to change the axle ratio if you select 20" wheels. I don't know if that was the case in 2013 when we bought ours, but it's possible that we just didn't catch that. I ended up with 20" wheels and 3.55's.
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Old 09-28-2015, 06:15 AM   #10
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Quote:
Originally Posted by lynnmor View Post
Ford used the 3.55 gears with the 20" wheels. The 3.31 gears with the 18" wheels turned almost the same RPM's. So as you decide on what to buy, compare apples to apples. I chose the snow plow package because it gave me the 3.55 gears with the 18" wheels, I don't know if any of this changed recently.

Folks like to throw out gear ratios without looking at the whole picture. Keep in mind that the diesel transmission has 5th gear as overdrive and 6th gear as over-overdrive. You can't compare gear ratios to another truck that uses a different transmission or different tires.

I doubt that you will find much difference in fuel usage with any choice you make. I like my setup as it is better for the transmission and shifting is reduced.
Ford build and price offers 17" and 18" wheels only on the F350 SRW with both 3.31 an 3.55, no 20" offered until DWR. With the F350 I am overbuilding my vehicle as I will be hauling a fifth wheel 10,500 loaded. Very little cost diff between 250 and 350 comparatively so taking the 350 because you can always go bigger with trailer if want. Also, just realized the Camper Package is not offered with the 3.31
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Old 09-28-2015, 08:58 AM   #11
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My F250 diesel runs 2000 rpm at 70 mph. I pull a 27' 5er and basically never downshifts. But, I run at 62 mph all,day long.
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Old 09-28-2015, 09:57 AM   #12
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My 2008 F-250 came with 18" wheels and 3.55 gearing. When I put on the 20" wheels the 3.55 gearing was too high for my towing needs in the mountains and elevation, so I had 4.10's installed before buying the 5th wheel. Best decision I ever made. Tows much better than before and the mileage didn't suffer that much.
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Old 09-28-2015, 12:10 PM   #13
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Quote:
Originally Posted by AJP207 View Post
Ford build and price offers 17" and 18" wheels only on the F350 SRW with both 3.31 an 3.55, no 20" offered until DWR. With the F350 I am overbuilding my vehicle as I will be hauling a fifth wheel 10,500 loaded. Very little cost diff between 250 and 350 comparatively so taking the 350 because you can always go bigger with trailer if want. Also, just realized the Camper Package is not offered with the 3.31
Lariat, KR, Platinum all have 20" options on SRW trucks. DRW 350's are 17" only and have either 3:73 or 4:30 gears. The camper package is not available with 20" wheels either.

If it were me, I'd order the 3:55's with the locker. Great middle of the road setup.

If you're ordering, then sure, go for the SRW 350. If you find what you want on the lot in a 250, don't shy away. A 250 with the camper package is 100% identical in every way to a SRW 350 but is de-rated by Ford to fill a different market segment.
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Old 09-28-2015, 02:25 PM   #14
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You will pick up 1 to 2 gym but I would go with the 3.55. It makes a big difference in what you can pull later on if you upgrade.
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