Switching to 3.55 or 3.73

[jrussell9109]

My truck stock runs a 3.31 gear and 32" tires. I installed 34" tires and did the conversion to get my truck back to stock gearing so there's no extra stress I need to go to a 3.55, I'm thinking if I'm gonna make the jump I may as well go to a 3.73, question is does the dealer then reprogram to accommodate going to a 3.73 since my new stock setup technically should be a 3.55 without any reprogramming.

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[Rockman5159]

My truck stock runs a 3.31 gear and 32" tires. I installed 34" tires and did the conversion to get my truck back to stock gearing so there's no extra stress I need to go to a 3.55, I'm thinking if I'm gonna make the jump I may as well go to a 3.73, question is does the dealer then reprogram to accommodate going to a 3.73 since my new stock setup technically should be a 3.55 without any reprogramming.

I hace a 2021 FX4 with 5.0 ten speed automatic 33" tires and 3:73 gearing and I absolutely LOVE it. IMO 3:73 way to go

 

[TMCCOY]

3.31 to 3.55 is a 7.2% increase in ratio. Hardly seems worth the expense unless you feel like the trucks performance has suffered too much. I would think with the 10 speed, it handles the larger tire fine. Going to 3.73 is a 12.6% increase. If set on making the change, I probably would go to 3.73 and have the dealer make the adjustment (call to confirm they can make the change).

 

[jrussell9109]

I think when I go down to order mud guards at the dealer tomorrow I'll see if I can get a quote on the switch to the 3.73 and what all it entails. Think I'll be much happier with acceleration and all the hills I drive

 

[TMCCOY]

A dealer would be the most costly option, a good 4WD shop could be considered. 3.73 Would be my choice.

 

[Mtnman1]

My truck stock runs a 3.31 gear and 32" tires. I installed 34" tires and did the conversion to get my truck back to stock gearing so there's no extra stress I need to go to a 3.55, I'm thinking if I'm gonna make the jump I may as well go to a 3.73, question is does the dealer then reprogram to accommodate going to a 3.73 since my new stock setup technically should be a 3.55 without any reprogramming.

Will have to go aftermarket for 3.73's.

The 3.31 and 3.55 is the 8.8 rear end. The 3.73 is the 9.75 rear end.

 

[jrussell9109]

Will have to go aftermarket for 3.73's.

The 3.31 and 3.55 is the 8.8 rear end. The 3.73 is the 9.75 rear end.

Thought I read the 9.75 is the 12 bolt pattern which mine is 12 bolt but a stock 3.31

 

[UPTHECREEK]

My truck stock runs a 3.31 gear and 32" tires. I installed 34" tires and did the conversion to get my truck back to stock gearing so there's no extra stress I need to go to a 3.55, I'm thinking if I'm gonna make the jump I may as well go to a 3.73, question is does the dealer then reprogram to accommodate going to a 3.73 since my new stock setup technically should be a 3.55 without any reprogramming.

Jrussell, Just curious of what motor your truck has ?

 

[jrussell9109]

Jrussell, Just curious of what motor your truck has ?

5.0

 

[UPTHECREEK]

5.0

I'm running 295/65-20 (35.1 tall) on my factory 20" wheels with 3.55s with the 3.5 EB and it turns them pretty well. I wanted 3.73s when I ordered the truck but wasn't available with my options.

 

[Mtnman1]

Thought I read the 9.75 is the 12 bolt pattern which mine is 12 bolt but a stock 3.31

All 3.31 are 8.8 rear end. All 3.73 are 9.75 rear end.

If you are 3:31 stock, you will need aftermarket gear set for the 8.8.

 

[Rockman5159]

A dealer would be the most costly option, a good 4WD shop could be considered. 3.73 Would be my choice.

on the other hand,dealer installed wont affect warranty 3:73 definately

 

[Tomatoboy]

Thought I read the 9.75 is the 12 bolt pattern which mine is 12 bolt but a stock 3.31

A note: The “super 8.8” (available in -both- IRS and solid axle) was introduced in 2015 for Mustangs and F-150s and includes larger bearings, a longer pinion with larger nut, 34 spline axles or half shafts, and most importantly for our discussion, a 12 bolt pattern. So you can’t tell them apart solely by the number of bolts anymore.

You can tell them apart by cover shape, however, as the original 8.8 had a rounded square shape and the 9.75 has sort of a tapered reverse D shape, and the super 8.8 has sort of the same rounded square shape of the original 8.8, just with a *slight* taper to the top, but keeping the flat bottom of the 8.8 that’s very unlike the 9.75.
Picture of old 8.8 cover https://www.f150hub.com/drivetrain/ford-8.8.html
Picture of new super 8.8 cover shape, aftermarket design but the pattern is the same: https://yourcovers.com/pml-ford-88-super-12-bolt-differential-cover.html
Picture of 9.75 https://www.f150hub.com/drivetrain/ford-9.75.html

So to tell them apart easily, one really just needs to check if the bottom of the cover is flat or not. If the bottom is tapered, you’ve got a 9.75. If it’s flat, it’s not a 9.75.

 

[pavementends]

Since your truck is a 4x4 I'd budget at least $2k to have the front diff regeared too - rear is fairly easy, front is a specialty service I've been told

 

[hotrodmex]

3.55 is the 8.8 rear end.

Not a blanket statement you can make. 3.55 also comes in 9.75; I have one.

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