Axle Ratio
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New User
Joined: Nov 2017
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Axle Ratio
I'm considering buying a 2017 F250 with the diesel engine but I'm not sure what axle ratio is best. I currently drive 5 miles to work but I will soon be retired and plan to travel with a 5th wheel. What axle is best for me? One vehicle I see online has the 3.55 ELA with LT275/65R 20E tires.
Cargo Master
Joined: Oct 2011
Posts: 2,958
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From: Denver, NC
See if the MODS can move this to the SuperDuty Forum for ya 
I will say this though, my dad's Diesel & the 06' F250 Diesel that I owned, has/had 3.73 Gears & IMO were perfect! My dad pulls flat-bed & 5th-Wheel, while I pulled mainly a 5th-Wheel all over. My 06' w/3.73's would pull over 10k without breaking a sweat & still maintain 10.5-11 mpg
I will say this though, my dad's Diesel & the 06' F250 Diesel that I owned, has/had 3.73 Gears & IMO were perfect! My dad pulls flat-bed & 5th-Wheel, while I pulled mainly a 5th-Wheel all over. My 06' w/3.73's would pull over 10k without breaking a sweat & still maintain 10.5-11 mpg
More Turbo
Joined: Sep 2016
Posts: 618
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Your choices are 3.31 or 3.55 w/6.7L.
I (unfortunately) managed to snag a F-350 with 3.31s and pull a 16k 5th wheel. Honesty it tows fine. Lotta power here. Way better the my 6.0 with 3.73s. However, I wish I had gotten the 3.55s just in principle.
I (unfortunately) managed to snag a F-350 with 3.31s and pull a 16k 5th wheel. Honesty it tows fine. Lotta power here. Way better the my 6.0 with 3.73s. However, I wish I had gotten the 3.55s just in principle.
Logistics Pro
Joined: Apr 2017
Posts: 3,674
Likes: 51
From: SoCal
You can't get 3.73s in a 250 diesel. 3.55 is great for towing, 3.31 gets a little better mpg.
To be honest, there's not that much difference between the two and with 925lbs of torque, who cares really
To be honest, there's not that much difference between the two and with 925lbs of torque, who cares really
Senior User
Joined: Mar 2017
Posts: 416
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From: Texas
If you want 20 inch wheels from the factory the 3.55 is required.
You have the choice if you get 17 or 18 inch wheels.
If you want 20's or a Platinum you don't have to worry about it Ford gives you the 3.55.
Ford rates both 3.31 and 3.55 for the same maximum towing weight where the lower gears on dually and gas trucks get higher trailer weight ratings.
That tells me they don't see much difference in the two choices.
I have two 2017 6.7 diesels both ccsb trucks.
A 250 with 18" wheels and 3.31 and a 350srw with 20" wheels and 3.55 I don't notice any difference. I'm sure if you measured 0-60 there would be a small difference between two otherwise identical trucks, but it's not noticeable in daily driving.
The 250 is averaging 15.9 over the first 4700 miles and the 350 is averaging 14.6 for the first 850 miles calculated with fuel buddy app.
Both see the same mix of city and highway driving.
You have the choice if you get 17 or 18 inch wheels.
If you want 20's or a Platinum you don't have to worry about it Ford gives you the 3.55.
Ford rates both 3.31 and 3.55 for the same maximum towing weight where the lower gears on dually and gas trucks get higher trailer weight ratings.
That tells me they don't see much difference in the two choices.
I have two 2017 6.7 diesels both ccsb trucks.
A 250 with 18" wheels and 3.31 and a 350srw with 20" wheels and 3.55 I don't notice any difference. I'm sure if you measured 0-60 there would be a small difference between two otherwise identical trucks, but it's not noticeable in daily driving.
The 250 is averaging 15.9 over the first 4700 miles and the 350 is averaging 14.6 for the first 850 miles calculated with fuel buddy app.
Both see the same mix of city and highway driving.
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Hotshot
Joined: Oct 2015
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From: Wyoming
When I ordered I specified the 3.55 axle ratio. I tow in Colorado and Wyoming so that extra bit of ratio over the 3.31 will come into play when I'm going up the high elevation passes at a steep grade. I went with the 18" wheels. There is a slight difference in wheel diameter between 18" and 20" that give me a bit more power to the ground. Plus, if you order the camper package you'll be forced into the 18" wheels. I was OK with that as I wanted the rear sway bar that comes in the camper package.
Tuned
Joined: Jun 2010
Posts: 317
Likes: 1
The only choice is 3:55 or 4:10 for diesel engine
More Turbo
Joined: Mar 2005
Posts: 522
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It's only .24 difference, not much at all honestly. About 6-7% difference on RPM, so you will be at 2000 vs 2130. I doubt 130 RPM will make much mileage difference IMO.
7000-8000 lbs 900+ tq truck mileage will not likely be affected much by this small change.
The 20' tires are a taller tire and the FX4 18" tires are slightly taller than the non-FX4 18's
275/65/18 Non FX4 101" circumference
275/70/18 FX4 104" circumference
275/65/20 107" circumference
So the 20's with 3.55 will be about the same RPM as the non- FX4 18's with 3.31.
So to really make you gear choice easier, figure out if you want which of the above tires you will be getting first then figure your gear choice.
Too confusing? Yes, and as I said I don't think these trucks really care about a .24 gear difference for mileage. For heavy towing, I like the lowest gear possible. In this case, 3.55
Now if the SRW had 3.31, 3.55, 3.73, and 4.10 to choose from, then I'd agree the 3.31 will probably get slightly better mileage than the 4.10.
7000-8000 lbs 900+ tq truck mileage will not likely be affected much by this small change.
The 20' tires are a taller tire and the FX4 18" tires are slightly taller than the non-FX4 18's
275/65/18 Non FX4 101" circumference
275/70/18 FX4 104" circumference
275/65/20 107" circumference
So the 20's with 3.55 will be about the same RPM as the non- FX4 18's with 3.31.
So to really make you gear choice easier, figure out if you want which of the above tires you will be getting first then figure your gear choice.
Too confusing? Yes, and as I said I don't think these trucks really care about a .24 gear difference for mileage. For heavy towing, I like the lowest gear possible. In this case, 3.55
Now if the SRW had 3.31, 3.55, 3.73, and 4.10 to choose from, then I'd agree the 3.31 will probably get slightly better mileage than the 4.10.
New User
Joined: Nov 2017
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^ This. The ratio of the gears in the rear differential is only part of the equation. What matters is the final drive ratio. Overall tire/Wheel diameter plays an important role as well. 3:31 with a shorter tire may have the same final ratio as 3:55 with a taller tire. There are a lot of free final drive ratio calculators on the net.