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Old 05-14-2015, 09:37 PM   #1
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Rear End Question

No, not that type of rear end. I'm referring to axle ratio.
I have a 2005 F350 5.4 L gasser to pull my fifth wheel. It pulls OK, but has a hard time passing gas stations. I am considering a newer truck, and have been considering diesels. However, it seems to me, that there have been some considerable improvements in gas engine towing capacities, so I may stick with gas. There are advantages to both.
However, the capacities can vary considerably depending on axle ratio. Ford has a choice of 3.73:1 or 4.30:1, which is approximately 15% difference. So, in theory would that translate to a 15% difference in fuel economy? (With 3.73 getting better mileage) How about in actual numbers? Manufacturers don't seem to report fuel economies for 3/4 or 1 ton trucks, so can't even compare theoretical factory mileages.
Any thoughts are appreciated.

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Old 05-14-2015, 09:53 PM   #2
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As a general rule, with mostly towing and mountains, get 4:30
Light loads at hwy speeds, get 3.73
I will be full timing with a 14,000 5th wheel. I ordered 4.30

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Old 05-15-2015, 03:04 AM   #3
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My old 250 would pass everything but the gas station. If i was lucky it would get 8mpg towing a Cub. 5.4 is a good engine, but its not really ideal for towing, it has issues just moving the truck around. I would suggest a larger engine or go diesel. You can't beat the diesel in the mountains.
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Old 05-15-2015, 05:37 AM   #4
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Beware of the f250 crew cab SWB diesel, it only carries 26 gallons... The f350 has 36. For the diesel, the 4.3 gear is only available on the f450.
For the gasser, the difference between 3.73 and 4.3 is basically 1 tranny gear... Ie driving in 5th vs 6th. My f150 5.0 had no problems going up 10% grades pulling 7000lbs with the 3.73 running strong at 4000 rpm, so I went with the 3.73 for my 6.2L gas f250. I have more range than a diesel f250 in the same configuration due to the 36 gal tank.
It's also my daily driver, so it keeps the revs down on the highway when not towing.
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Old 05-15-2015, 07:51 AM   #5
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The 4.30 gear is available on the f-350 diesel when you choose the "high capacity tow package" , which includes the wide front axel for smaller turning radius

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Old 05-15-2015, 09:52 AM   #6
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Originally Posted by marcham View Post
Beware of the f250 crew cab SWB diesel, it only carries 26 gallons... The f350 has 36.
Inaccurate info. All 2015 SuperDuty pickups with gas engine get the 35-gallon gas tank.

Diesel fuel tank size is related to bed length, not model. 8' bed gets the 37.5 gallon diesel fuel tank on F-250, F-350 SRW and F-350 DRW. The 26 gallon diesel tank is on the SuperDuty SRW pickups with diesel engine and shorty bed. (Ford no longer makes a shorty dually, so all DRWs have the big fuel tank.)

Originally Posted by chefbob
The 4.30 gear is available on the f-350 diesel when you choose the "high capacity tow package"
Misleading info . The high capacity tow package is available only on a dually 4x4 CrewCab diesel, and requires the 4.30 axle. But the 4.30 axle is available on any F-350 DRW, gas or diesel. The high capacity Tow pkg is not available with the gas engine.

Originally Posted by Tollyman
Ford has a choice of 3.73:1 or 4.30:1, which is approximately 15% difference. So, in theory would that translate to a 15% difference in fuel economy?
Only when the engine is coasting along with no load and moderate speed, so the engine is not working up a sweat.

If you're going to tow more than a rowboat with a 2015 gas engine in an F-350 DRW, then you want the 4.30 optional axle ratio. Unloaded MPG will be slightly better with the 3.73 axle, but towing MPG will be worse because the engine has to work harder.

The new 6.2L gas engine is a lot more engine than the 5.4L in your old truck. If you are happy with the towing performance of your 2005 with the 5.4L, then I suspect you'll be esthetic with the 6.2L and 4.30 axle.

But most people would not be happy towing a heavy trailer with an F-350 DRW with the gas engine. The diesel has so much more torque that it's a pleasure to tow a heavy trailer up a mountain pass. If you spring the big bucks for the diesel dually, then be sure to also order the high capacity towing package. You'll LOVE it!
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.
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Old 05-16-2015, 08:37 PM   #7
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The brand of truck will play a big part on determining gearing. Google the particular transmission gearing for the truck which your interested in. Will tell you, the Ram brand will require a lower gearing on the diff as the transmission gearing is significantly higher. Starting at approx. a 3.7 compared to Ford and GM with something in the 4's. Big difference in the higher elevations, especially when towing heavy and at slower speeds. Get your trans model and Google the gearing. You'll be surprised.
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Old 05-16-2015, 09:55 PM   #8
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I have a diesel with 4:30 axle ratio. The truck tows a 15,000lb 5th wheel. I get 10.2 mpg on average towing. 9.5 towing into the wind and 11 mpg towing with the wind.

What happens a lot with my truck is when I encounter a uphill the truck increases turbo boost and does not down shift out of 6th gear or does not down shift much.

I wish I had a 3:73 axle ratio when running empty and a 4:30 axle ratio when towing.

If you are towing with the 5.4 gas engine you would be amazed with the newer diesel engines. I would try to move up to any 2011 truck gas or diesel. There were improvements made to Ram, Ford, and Chevy. Ford...new powerful engines, Chevy...new heavier frame, Ram...new design. All 2011 trucks were very nice.

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