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#4666131 - 02/14/18 06:57 PM 3.55 vs 3.73 ?
lawnguy Offline


Registered: 08/17/14
Posts: 320
Loc: Florida
Hello Fellas, I provided my VIN# to my local Ford dealer,and they say that my 2000 F150 4.6 V8 4x2 has 3.55 gears.
I don't understand what all that means honestly,but I was told if I replace the 3.55 to a 3.73 I will gain in 0-60 times,etc.
Will this swap affect my interstate gas mileage,and how by much if so? Also, will this change affect my speedometer in any way?

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#4666140 - 02/14/18 07:05 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
SilverFusion2010 Offline


Registered: 06/29/11
Posts: 1528
Loc: Crawfordville FL
Your gas mileage will go down and your speedo will read low.

3.55 is the gear ratio. 3.55 rotations of the drive shaft cause the rear axle to rotate once.

So 3.73 gears will mean a higher rpm at all speeds, using more fuel, and the speedometer will be off a few percent
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#4666143 - 02/14/18 07:08 PM Re: 3.55 vs 3.73 ? [Re: SilverFusion2010]
lawnguy Offline


Registered: 08/17/14
Posts: 320
Loc: Florida
Thanks for the info.

Originally Posted By: SilverFusion2010
Your gas mileage will go down and your speedo will read low.

3.55 is the gear ratio. 3.55 rotations of the drive shaft cause the rear axle to rotate once.

So 3.73 gears will mean a higher rpm at all speeds, using more fuel, and the speedometer will be off a few percent

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#4666150 - 02/14/18 07:20 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
BrocLuno Offline


Registered: 09/06/15
Posts: 5226
Loc: Kalifornia Kollective
It all depends on the rolling diameter of the tires. Lets say you started with 235/70R15 (just an example) and changed up to 31-10.50 R15. The increase in tire size would hit as it lowers the effective "leverage" of the system so you feel less acceleration and somewhat doggy motor response, the speedo reads slow and your gas mileage may suffer becasue you are into the throttle to make the bigger tire roll into wind resistance ...

Now let's install 3.73 gears and you will find that you are back to even. You have higher ground clearance and more load capacity inherent in the larger tires, but the motor feels more responsive and the speedo is back to where it should be smile

It's all about combination. No single factor. Doing gear calcs w/o knowing OEM tire sizes and current tires sizes is like guessing in the dark ...


Edited by BrocLuno (02/14/18 07:22 PM)
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#4666153 - 02/14/18 07:23 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
14Accent Online   content


Registered: 08/03/17
Posts: 334
Loc: MN
Stock for stock, 3.73 gears would be a good upgrade in that truck. 4.10's would be more fun, however.

You could probably leave the computer alone just going to 3.73's, 4.10's I'd recommend getting a tune for the transmission to optimize shift points and correct the speedo.

Those 4.6 trucks really do feel like dogs with the long gears in the 4r70 trans and tall rear ends. All about fuel economy.

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#4666155 - 02/14/18 07:26 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
lawnguy Offline


Registered: 08/17/14
Posts: 320
Loc: Florida
I currently have 245/70R16 106T Tires,and 16.8 custom wheels. I like a lower stance tan a tall truck. Thats just me.

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#4666172 - 02/14/18 07:41 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
Srt20 Offline


Registered: 07/14/14
Posts: 478
Loc: Wisconsin
Going to a 3.73 gear will raise your RPM about 120 rpm, leaving everything else the way it is.

Tires are a different story.


Edited by Srt20 (02/14/18 07:42 PM)

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#4666174 - 02/14/18 07:46 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
JLTD Online   content


Registered: 12/15/17
Posts: 380
Loc: US
I'd expect no more than 1mpg difference, and think you'll probably like the 3.73s better.

edit: I know someone that has had both in a Ford pickup and this is what was relayed to me


Edited by JLTD (02/14/18 07:49 PM)
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#4666187 - 02/14/18 07:58 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
Alex_V Offline


Registered: 04/10/16
Posts: 1117
Loc: Campbellsville, KY
The number (3.55, 3.73, etc.) refers to the gear reduction ratio of the axle or axles - higher the number, the faster the engine spins at the same ground speed. It will make the truck feel slightly more powerful especially at lower speeds, and it will tow a given load with a bit more ease. However while not a guarantee you may see a moderate loss in gas mileage and unless corrected the speedometer will be off.
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#4666189 - 02/14/18 07:59 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
JohnnyJohnson Offline


Registered: 10/22/09
Posts: 2382
Loc: Wet side WA
3.73 my guess is about a 10% loss in fuel mileage.
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#4666197 - 02/14/18 08:03 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
clinebarger Offline


Registered: 12/19/13
Posts: 1747
Loc: Fort Worth, Texas
3.55:1 means for every single revolution of the drive shaft.....The axles will turn 3.55 revolutions.


I'm guessing you have a 4R70W transmission with a 0.70:1 overdrive ratio, I don't see you having much MPG loss with 3.73:1 rear gears. Unless your running shorter tires than stock.

This WILL NOT gain enough performance wise to justify the cost unless you buy a complete differential for a good price. You will need the calibration changed for trans shift points/TCC scheduling & the Speedometer/Odometer.

4.6L Modular's don't make good full size truck engines.....Neither does the Chrysler 4.7L, GM 4.8L, or the Toyota 4.7L/4.6L.
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#4666198 - 02/14/18 08:04 PM Re: 3.55 vs 3.73 ? [Re: JohnnyJohnson]
Rand Offline


Registered: 08/20/03
Posts: 12487
Loc: NE,Ohio
Originally Posted By: JohnnyJohnson
3.73 my guess is about a 10% loss in fuel mileage.


uh no not even 5%

its a very small difference not even worth doing 3.55 to 3.73

That is a 5% increase in RPM at any given speed (torque converter locked)

Which doesnt translate into %5 less fuel economy.
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#4666204 - 02/14/18 08:12 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
eljefino Offline


Registered: 06/15/03
Posts: 32379
Loc: ME
You'd barely notice the change. I'd go for something 3.9ish or even 4.11.

Driver's door sticker should have a gear code if you don't trust your dealer.

If you're used to a certain engine speed on the highway... eg your ears hearing the pitch of the noise... you might subconsciously slow down so you turn the same RPMs. This would actually save way more fuel if you could pull it off.

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#4666208 - 02/14/18 08:15 PM Re: 3.55 vs 3.73 ? [Re: clinebarger]
OVERKILL Offline


Registered: 04/28/08
Posts: 35986
Loc: Ontario, Canada
Originally Posted By: clinebarger
3.55:1 means for every single revolution of the drive shaft.....The axles will turn 3.55 revolutions.


I'm guessing you have a 4R70W transmission with a 0.70:1 overdrive ratio, I don't see you having much MPG loss with 3.73:1 rear gears. Unless your running shorter tires than stock.

This WILL NOT gain enough performance wise to justify the cost unless you buy a complete differential for a good price. You will need the calibration changed for trans shift points/TCC scheduling & the Speedometer/Odometer.

4.6L Modular's don't make good full size truck engines.....Neither does the Chrysler 4.7L, GM 4.8L, or the Toyota 4.7L/4.6L.


This. You will not make big gains going up such a small ratio change. The 5.0L Mustangs came with 2.73's or 3.08's stock, 3.73's on them was a pretty decent jump, 3.55's are so close that it would be silly, you'd have to go 4.10's or 4.56's to get a dramatic difference and then you'll be increasing your operating RPM, which may affect fuel mileage.

I had an '88 F-250 2WD with the 300 I6 backed by a C6 with 4.10's out back. It was winding to the moon on the highway at 120Km/h with no overdrive but would pull the proverbial [censored] out of the world. The truck worked quite well however when I swapped in a 302HO, which was much happier in the upper RPM range.
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#4666224 - 02/14/18 08:31 PM Re: 3.55 vs 3.73 ? [Re: lawnguy]
Blkstanger Offline


Registered: 08/13/11
Posts: 1901
Loc: Lakeside, CA
3:73s is not worth the effort. Go 4:10s if you want to wake that truck up. That engine trans combo works well with 4:10s. It would tow much better too.
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