Which gear 3:31 or 3:55? - Ford F150 Diesel Forum
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post #1 of 9 (permalink) Old 02-15-2019, 07:01 AM Thread Starter
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Which gear 3:31 or 3:55?

Hi, I'm looking at a good deal on a demonstrator 2018. It would primarily be used as to replace an aging 2500 Suburban, mainly highway driving. This unit has standard locking 3:31 gears. I have looked and not found much discussion on which gear set people are most pleased with. If I were pulling an RV a lot, 3:55 might be a better choice, but I won't be. The new 10-speed transmission (I've read) makes even a steeper gear like 3:31 better than it used to be. Diesels always seem to have their "sweet spot" in the RPM range and was curious of others experiences. Maybe which gear set you have and RPMs at various highway speeds? Thanks
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post #2 of 9 (permalink) Old 02-15-2019, 07:18 AM
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Depends on you really my truck has 3.31 gears and at most will haul 4 wheelers and a flat bottom boat. I average around 24 mpgs and a 600 plus mile range, imo the 3.31s are great got sub 10 thousand pound hauls and on the highway unloaded. If you live in a mountainous area and haul over 10 thousand pounds regularly then I would say get the 3.55s. If not then the 3.31s are great
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post #3 of 9 (permalink) Old 02-15-2019, 07:29 AM
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@mascrappo - I have a 2018 Platinum w/FX4 package that automatically gives you the 3.31 screw w/Electronic Locking Axle. I didn't buy mine for towing anything major, as I tow a small 16' enclosed trailer occasionally.

At 65MPH on a flat highway, I am always amazed when I look down and see the tach purring along at exactly 1600PRM and the 10-speed transmission in 10th gear. I now understand why they put this smallish "BabyStroke" 3.0L V6 engine in the F150 vs something much larger, louder, and heavier like their SuperDuty "big brothers", as I think the powertrain is the right balance of low-end torque to get things moving and balanced gearing to sip the fuel once you are on the highway = these were built for fuel efficiency. I thought the 3.0L might be a little smaller than I would have like, but knowing the Dodge used the same displacement in their EcoDiesel and now driving my F150 Diesel for 6 months, I have no qualms about the power of the 3.0L PowerStroke.

I saw your earlier posts about worrying about the EGR recall -- please understand that you are also getting a standard 5yr/100K Powerstroke warranty on your engine, instead of a standard 5yr/60K Powertrain warranty on a gasoline engine.

Good luck with your decision.

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Last edited by jmperlik; 02-15-2019 at 07:33 AM.
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post #4 of 9 (permalink) Old 02-15-2019, 08:06 AM Thread Starter
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jmperlik thank you for the RPM numbers. I have even watched the numerous YouTube reviews and tried to see the gauges, but figured out it would not do me any good if I don't know what axle they had. I have a Super Duty and it can do the heavy work! Thanks again.
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post #5 of 9 (permalink) Old 02-15-2019, 08:09 AM Thread Starter
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Quote:
Originally Posted by jamanrr View Post
Depends on you really my truck has 3.31 gears and at most will haul 4 wheelers and a flat bottom boat. I average around 24 mpgs and a 600 plus mile range, imo the 3.31s are great got sub 10 thousand pound hauls and on the highway unloaded. If you live in a mountainous area and haul over 10 thousand pounds regularly then I would say get the 3.55s. If not then the 3.31s are great

Thanks jamanrr, not going to be a heavy hauler, mostly 75mph down the highway on flat land. I've noticed even the Super Duty is going to taller gearing, without issue. Thanks again.
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post #6 of 9 (permalink) Old 03-06-2019, 01:51 PM
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love my 3.31

I may have different use for my truck than many here, as I will not use it very much for towing. I have a 99 Dodge dually with 24 valve Cummins to tow my RV, so that's not an issue.

I bought this truck as a retirement truck. Maybe some light towing, but more for travel comfort and mileage. I got a rear wheel drive with the 3.31 to help mileage wise, and it has proven to be a good choice. Not many miles on it yet, but about 30.5 mpg combined so far. It is amazing on the highway ... very quiet and plenty of quickness.

I drove a 3.0 with the 3.55 ratio, and it had a bit more off the line quickness, but not really very noticeable. I have heard it will cost 1-2 mpg to get the 3.55, and don't have any info about the top ratio available on these trucks. The biggest mpg cost difference would be the 4x4 vs 2x4.
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post #7 of 9 (permalink) Old 03-06-2019, 02:53 PM
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I went 4x4 /3.55. I live in the San Joaquin Valley of Ca - its FLAT- but have a second home in the Lake Tahoe area. I love how the 3.55's perform in the mountains. So far 3 weeks in averaging 27-28
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post #8 of 9 (permalink) Old 03-06-2019, 08:57 PM Thread Starter
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I have been very pleased with 3:31. So far with under 1500 miles, I average 25mpg overall with the 4 x 4. Dealer said it could get better as engine gets "loosened up". I would not have passed over the 3:55, but thus far, the standard axle has been very good.
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post #9 of 9 (permalink) Old 03-08-2019, 08:18 AM
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If you think you may swap out for larger tires down the road, or do a lift/level and larger tires, then I would go with the 3.55 gears for rear end axle ratio.

My two required options for the 2018 F150 3.0 KR were the tow mirrors and the 3.55 rear axle ratio. Yes, with the 10 speed tranny the 3.31 gears will tow just fine for the ~4,200 pound boat I will be towing around; however, I hate the look of the 275/55R20 tires--not enough sidewall rubber for me, so the next tires I put on will be 275/60R20 (believe me that my eyes can see a big difference in the two...many of the Ram trucks come stock with 275/60R20). The larger tires have the effect of reducing the rear axle ratio, in this case from 3.55 to 3.43, and I wouldn't want to reduce a 3.31 to ~3.19 on this truck. And, the 275/60R20's can be done without a lift or leveling with no rub.

The following website did the math for me...

DML Tire and Wheel Calculator "http://www.dakota-truck.net/TIRECALC/tirecalc.html"

Anyone that is considering changing tire sizes can use the above website to understand the impacts of doing so. You can then use FORScan to recalibrate and correct for the speedometer/odometer error (see spreadsheet in below link)....

In BCM (BdyCM) 726-12-01 xxxx 09F2 xxxx
should be the starting setting for 275/60R20—refer to the spreadsheet for an alternative approach to fine tuning based on speedometer reading vs GPS actual
https://docs.google.com/spreadsheets...730/edit#gid=0
https://docs.google.com/spreadsheets...#gid=110328400
Attached Images
File Type: png TIRECALC.png (335.8 KB, 5 views)

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Last edited by kdjasper; 03-08-2019 at 08:27 AM.
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