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Old 08-26-2014, 05:59 PM   #1
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Gear ratios

I have a 28.5 cougar right now , but could see the possibility of a bigger unit down the road . we are looking to buy an f-350 w/ 6.7 diesel . One of the things they ask is what gear ratio for the axles . stock seems to be 3.31 with the next step being a 3.55 . are the lower gears really necessary with the added torque from the diesel? Does anybody have experience with mileage differences between the two ratios? Thanks.
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Old 08-26-2014, 06:09 PM   #2
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There will be different tow ratings for the two gear sets listed in the owners manual or the online paper work, could be 2, 000 #+ more for the higher numbered gear. There would be a 10% increase in RPM at cruising speed; but dropping your speed back should amount to the same MPG.
My first tow vehicle was geared wrong, 3.54 vs 3.91option, lived in second gear on hills and that sucked fuel, didn't make that mistake again.
Second truck had 4.10 seldom dropped lower than 3rd ( direct ) and got 1 1/2 mpg better towing than the first. Even though the trailer was 2,500# heavier.
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Old 08-26-2014, 06:27 PM   #3
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Skip , what kind of mileage were you getting with the 4:10 while towing?
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Old 08-26-2014, 06:41 PM   #4
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My mileage on that particular truck wouldn't mean much to what your looking at.
3/4 ton Dodge 4x4 , 8.0l V-10, trailer weighed 9200# , I-5 in CA, where towing speed is limited to 55MPH. I'd set the cruise control at 59 and get 12.7 mpg. Anywhere I'd get the speed up to 65 that would drop to just under 12. Running empty best I ever got was 17. Without the trailer pushing it down hill, that truck wouldn't coast down a flight of stairs.
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Old 08-26-2014, 08:06 PM   #5
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I have the 3.55 gears with 18 inch wheels. The lower gears are actually a bit better for the transmission and the engine braking effect. I doubt that you would waste enough fuel to matter.
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Old 08-26-2014, 08:20 PM   #6
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Lynnmor , how do you like the truck ? If you did it over would you buy another?
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Old 08-26-2014, 09:19 PM   #7
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Quote:
Originally Posted by jim81147
... we are looking to buy an f-350 w/ 6.7 diesel.
You should spell out that you're looking at the F-350 SRW, not the one ton dually.

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Originally Posted by Skip426 View Post
There will be different tow ratings for the two gear sets listed in the owners manual or the online paper work, could be 2, 000 #+ more for the higher numbered gear.
That might seem logical, but it's not true for the '11-up Ford F-350 SRW diesel. Both 3.31 and 3.55 axle ratios have 23,500 GCWR, so the tow ratings are the same.

With 17" or 18" tires, 3.31 ratio is standard, and you get 3.55 with an optional suspension pkg, either
1] camper pkg
2] Heavy Service front suspension, or
3] Snow plow prep pkg

3.31 and 3.55 axle ratios are your only choices with SRW diesel with 17" or 18" tires.

With DRW, you get 3.73 - period. 3.73 open axle is standard, and 3.73 limited slip is an option.
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Old 08-27-2014, 09:30 AM   #8
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I am not really sure , at this point , which rear wheel option that I am looking at . I cant see us ever having a huge , heavy 5er in the future , but who knows . I know that adding sliders adds tons of weight and we both like the added room that you get with them . On the other hand we like going to the national and state parks a lot and have seen many instances where the larger 5ers wont fit .
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Old 08-27-2014, 10:18 AM   #9
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Lynnmor , how do you like the truck ? If you did it over would you buy another?
I really like the truck. Plenty of power and good mileage. Only had a few minor problems that the warranty covered. Yes, I would buy another Ford but a KZ trailer, no way.
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