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What is needed to go from 3.15 to 3.73?

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Old 03-19-2017, 07:28 PM   #1
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Default What is needed to go from 3.15 to 3.73?

I have a 2013 Ford Ecoboost 3.5L 2WD Lariat Supercrew.

It has what is referred to on the original sticker as "max tow pkg" but the ratio is only 3.15. As far as I can tell, because it's a 2WD, the max tow refers to the built-in trailer brake, towing receiver etc. But the max towing is 8600 lbs I believe.

I want to tow a trailer that is 6k lbs dry. With the bedliner, tonneau cover, all our crap, our dogs and our butts, I'm figuring we are probably close to, or over, 8k lbs. We are planning a 4k+ mile road trip this summer that is going to take us over numerous mountain passes and through the Utah/Nevada desert in the summer sun.

I'm looking at upgrading to the 3.73 ratio in order to improve the towing capacity, but I'll be honest that I'm very new to this whole thing. I love my "new" truck, but I have only just begun the process of finding out all the amazing things I can do to upgrade it.

So, my question is what all has to be changed when doing the upgrade from 3.15 to 3.73? Is it really just the pinion and ring? Or are there more parts that I need to buy to make sure that I get everything needed before we start this process.

Appreciate the help guys! Looking forward to giving my truck some style, but really just wanting to ensure I don't white-knuckle my whole trip worrying about overheating my transmission.
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Old 03-19-2017, 08:09 PM   #2
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Best let a pro handle the job, it requires special tools.
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Old 03-19-2017, 08:16 PM   #3
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Oh I intend to do that, but I have one guy who sells the parts saying they cost about $350 - $450. But I'm seeing on american muscle and even Amazon that they ring and pinion range from $150 - $300. So I'm wondering if the guy has a whole package of additional parts that are necessary or if I'm just paying a lot for something when I can just order them from one of these online stores.
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Old 03-19-2017, 08:28 PM   #4
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Ford sells a kit for about $500, comes with everything needed. The gears, bearings, shims, etc.
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Old 03-19-2017, 08:53 PM   #5
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At least you're 2WD so the labor won't kill you as much.
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Old 03-19-2017, 10:16 PM   #6
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Wel before you go through all that expense, what is the payload of the truck, tongue weight of the trailer, and weight of all the gear and people in the truck?

Reason I ask, changing the gearing will not give the truck the ability to tow more trailer. Your limit is by the payload.

My last truck was a 14 EB with 3.15 gears and towed 6300# travel trailer just fine. Having shorter gears would ease the burden on the engine a bit, would run a bit cooler, but it still pulled it just fine.

Interesting that it has Max Tow with 3.15 gearing. Every build and price I have seen shows a requirement of 3.73 gears. Have you checked the door tag to see what the axle code is? A 3.15 will have an axle code of 15.
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Old 03-19-2017, 11:03 PM   #7
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Thanks acdii!

It does have the axle code 15. It has the 9.75 differential. According to the documentation I found it is indeed a 3.15 but it has the max payload package which gives it the HD load.

The total GAWR is 7650. The GVWR is 7100 lb.

I don't recall the tongue weight offhand and it's dark out now.

On the 2013 2WD Lariat the max tow package is weird. Has the trans cooler, hitch assembly, and brake controller. But just the 3.15 gears.
I'm not looking to tow more trailer. I'm looking to tow the 7k - 8k lbs without worry about handling or overheating the transmission.

From everything I'm hearing, the trick with these trucks is to keep the trans fluid cool. I'm adding the 170 tstat to help with engine cooling. I'm also adding some Roadmaster RAS springs to the back for some extra stability.
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Old 03-20-2017, 09:04 AM   #8
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Check the payload on the yellow sticker. If it is over say, 1600#, then go for the gear swap! Either 3.55 or 3.73 will do the trick. 3.55 is a compromise gear, gives you better towing without too much sacrifice of fuel economy when not towing. If my truck didn't get wrecked, that was one of the planned upgrades this year to put in an Eaton TrueTrak and 3.55 gears.

Since the 3.15 is an open diff, and you are planning a gear change, now is the best time to upgrade the diff itself with a Limited Slip one. From my research the best one to use is the Truetrak because it has no wear items in it and doesn't require friction modifiers.
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Old 03-20-2017, 09:11 AM   #9
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Yeah, the Payload sticker is going to tell the story. It doesn't change with different gearing. Lariat's are heavy, and you're likely going to discover that Payload is your limiting factor.

There is a Towing section of the forum that covers this in great detail. But find the Payload sticker first. https://www.f150forum.com/f82/
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Old 03-20-2017, 10:22 AM   #10
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Thanks guys! By yellow sticker, are you referring to the "Tire and Loading Information" sticker?

It says at the top that the occupants and cargo should never weigh more than 1385lbs

Not clear what that means for all this.
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