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Old 05-06-2009, 14:58   #1 (permalink)
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3.55 axle ratio and towing

Hi. My truck has the 3.55 axle ratio, wish it had the 3.73, I do tow a camper. Is it really that much of a difference? Is the different towing ratings for the 150's only because of axle ratio differences? I know the 3.73 will have more torque, but do I run the risk of damaging the diff. if I tow something fairly heavy with a 3.55? What would it take to change the 3.55 to 3.73? Is it just a matter of changing the gears inside the differential?

2005 F150 Super Crew, 5.4, 4X4
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Old 05-13-2009, 08:39   #2 (permalink)
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Re: 3.55 axle ratio and towing

The diff has the same strength regardless of gearing, but the gears themselves are stronger as the numbers go DOWN. Read your owner's manual - it should have a chapter dedicated to towing/hauling, and how to calculate the actual GCWR.

Changing the gears? Yes, and no. It's a very specialized task that requires the right experience & equipment. Read these:
http://www.fordracingparts.com/downl...And-Pinion.pdf
Differential Repair & Installation Instructions
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Old 05-20-2009, 17:53   #3 (permalink)
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Re: 3.55 axle ratio and towing

Going from 3.55 to 3.73 won't make much of a difference. If you're going to spend the cash on a gear swap, I'd advise going to a 4.10 ratio, especially if you plan to regularly with it. Highway mileage will drop as the highway rpms will be higher, but you'll be able to tow the maximum amount possible for your year and engine with 4.10 gears. Stepping up to a larger gear doesn't mean your towing capacity increases, as the transmission, chassis and suspension can only handle to much weight before parts begin to stress, and fail.

Lower gears allows easier starting, especially when pulling trailers. One benefit is you'll notice less gear hunting when climbing hills. Higher engine speed means less downshifting, which comes in handy when towing a load.

Don't rule out tires either. Depending on the weight of the trailer, cargo and passengers in the vehicle, you can run smaller diameter tires which will give better performance. If you run 32" tires, stock, running 30" XL rated tires (provided they fit on rim, etc) will allow the feeling of you having 3.73 gears.
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Old 05-21-2009, 21:36   #4 (permalink)
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Re: 3.55 axle ratio and towing

Quote:
Originally Posted by 360ci View Post
Higher engine speed means less downshifting...
While that's technically true, it's very misleading. It's possible for the engine to be OUT of its power band on the high side, too. So if you change the axle gears & find yourself at the rev limiter often, you haven't helped performance, reliability, or economy.
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...smaller diameter tires which will give better performance.
...if you consider lower speed to be "better".
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Originally Posted by 360ci View Post
...will allow the feeling of you having 3.73 gears.
Your math is a little off, and so is your theory. It won't be exactly the same - the driveshaft will be spinning 5% faster at the same actual speed, and the speedo will be +5% wrong.
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Old 05-22-2009, 04:59   #5 (permalink)
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Re: 3.55 axle ratio and towing

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Originally Posted by Steve83 View Post
While that's technically true, it's very misleading. It's possible for the engine to be OUT of its power band on the high side, too. So if you change the axle gears & find yourself at the rev limiter often, you haven't helped performance, reliability, or economy....if you consider lower speed to be "better".Your math is a little off, and so is your theory. It won't be exactly the same - the driveshaft will be spinning 5% faster at the same actual speed, and the speedo will be +5% wrong.
I wouldn't worry about going over the power band, unless he plans to drive on the interstate in second gear, or he has fitted his truck with a low revving diesel that he failed to mention. Raising highway rpms by a few hundred at the same speed by lowering the gear ratio a few percentage points won't be overly noticeable. It'd be the same as if driving on slightly shorter tires. Driving on shorter tires with the same axle ratio is nearly the same as driving on the same tires with a shorter axle ratio. Some speedometer calibration will have to be done irregardless, if he decides to change out the gears, or go with a different diameter tire. Most programmers, and dealers, can recalibrate the speedo - for a price. Obviously going with shorter gears on a 2wd is the lesser cost option, especially on a 2wd. Going from 3.55 to 3.73 won't even be noticeable, in acceleration or fuel economy. If he's serious about changing the rear gear, it's best to go with a 4.10, to make it worth while. That should, relatively, increase highway rpm in top gear by around 400rpm. If he's the type who tows a trailer at say 70mph, then the engine will be in the 2400-2600rpm range in third gear, which is the start of the powerband of the 5.4L. With the sufficient low end torque (350lb ft @ 2500rpm) of the 5.4L, he should only require a downshift to second on steep grades. Even in second gear at that speed he'll still be well within the powerband of the engine.
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