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Old 11-19-2012, 06:24 AM   #15
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2009 27' FB Flying Cloud
1991 35' Airstream 350
Jay , Oklahoma
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I have towed with 2 Duramax trucks, and loved them both. Both rear-end gears were 3.73.

The major difference is the transmisson in the '12 is a six speed. That brings the cruise RPM down to around 1800 at 70 mph.

I do hate the DEF issue, but I guess the folks at the EPA like having all the air scrubbers driving around. So we can have more iceburgs, of course.

Regards,

JD
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Old 11-25-2012, 11:25 PM   #16
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Fort Worth , Texas
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I wouldn't sweat 3.31. I intend to change my 3.73 manual transmission DODGE to 3.42. I don't mind going a bit slower from a stop or on ascents, and the gear ratios would allow me to otherwise (solo) to stay closer to peak torque in most situations (better mpg).

These trucks are all over-powered. Mine as much as yours despite the differences when it comes to pulling any size A/S.

Put your reading, instead, to a PRO PRIDE hitch and TT disc brake conversion.

.
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Old 11-28-2012, 09:04 PM   #17
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2002 30' Classic S/O
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3.73 all the way. I would have a 4.10 if they offered it but they don't. Most come with a 3.55 but you can find a 3.73 if you look hard. Ford wants more higher rear end ratios as it shows up better on their emissions(lower the number, the higher the ratio). Mileage is a wash between the 3.73 and the 3.55. Higher than that and you loose power but gain mileage, but also more strain on the tranny. Your choice.

Pap
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Old 11-28-2012, 11:00 PM   #18
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3.73 in our diesel and the AS disappears in the mirror ...
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Old 12-09-2012, 04:33 PM   #19
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1986 34' Limited
1966 24' Tradewind
Conifer , Colorado
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3.31 with18" wheels versus 3.55 with 20" wheels

I thank you Airstream truckers for your descriptions of your experience. I did some thinking on this axle ratio business. Tire Rack has a great website that gives tire size diameters. 275/70R18 tires are 33.2 inch diameter resulting in 627 revolutions to travel a mile, 275/65R20 tires are 34.1 inch diameter resulting in 610 revolutions per mile. The Super Duty transmission top gear is .67 to 1 overdrive. So 3.31 axle ratio results in a 2.21 overall ratio, and 3.55 axle results in 2.38 overall ratio.

The 3.31 gears with 18 inch wheels gives 1386 engine rpm at 60, and 3.55 gears with 20 inch wheels gives 1452 rpm at 60.

Diesels don't have throttles. I probably have to feed the same fuel at 1386 rpm as I would at 1452 rpm to maintain 60 with the Airstream behind me.

I think I am fussing about nothing. Either combination would result in the same performance.

David
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Old 12-09-2012, 07:20 PM   #20
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2000 34' Excella
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Dude I read the posts. No one mentioned that the new 2012 Ford diesels are spec'd at *800 ft-lbs* and 400hp. That is way over the top of what they have been the last few years. Airstreams are light in the big picture of 40' 5th wheels etc and the other junk people tow. Get some nice tall gears and enjoy the fuel mileage and cruising along. I have a 34' excella and had a Dodge cummins 3.73 with the factory 600ft-lbs. That towed just fine all conditions. I now have an '02 7.3 ford (550 ft-lbs with a standard) with 4.10 gears. It also tows fine and I wish I had 3.55s.
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