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3.55 vs 3.73 ? #4666131
02/14/18 07:57 PM
02/14/18 07:57 PM
Joined: Aug 2014
Posts: 326
Florida
lawnguy Offline OP
lawnguy  Offline OP
Joined: Aug 2014
Posts: 326
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?

Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666140
02/14/18 08:05 PM
02/14/18 08:05 PM
Joined: Jun 2011
Posts: 1,754
Crawfordville FL
SilverFusion2010 Offline
SilverFusion2010  Offline
Joined: Jun 2011
Posts: 1,754
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


2010 Ford Fusion SE 3.0L V6, 178k miles M1 HM 10w-30
Re: 3.55 vs 3.73 ? [Re: SilverFusion2010] #4666143
02/14/18 08:08 PM
02/14/18 08:08 PM
Joined: Aug 2014
Posts: 326
Florida
lawnguy Offline OP
lawnguy  Offline OP
Joined: Aug 2014
Posts: 326
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

Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666150
02/14/18 08:20 PM
02/14/18 08:20 PM
Joined: Sep 2015
Posts: 5,839
Kalifornia Kollective
BrocLuno Offline
BrocLuno  Offline
Joined: Sep 2015
Posts: 5,839
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 ...

Last edited by BrocLuno; 02/14/18 08:22 PM.

Formerly in marine engineering. In an earlier life I owned my own petroleum tank truck, and hauled for the majors and independent's.
Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666153
02/14/18 08:23 PM
02/14/18 08:23 PM
Joined: Aug 2017
Posts: 558
MN
14Accent Offline
14Accent  Offline
Joined: Aug 2017
Posts: 558
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.


2013 Chevrolet Volt - Saves enough in fuel to pay for itself!

2003 Cadillac Seville STS - The winter "beater", it uses the Volt's fuel
Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666155
02/14/18 08:26 PM
02/14/18 08:26 PM
Joined: Aug 2014
Posts: 326
Florida
lawnguy Offline OP
lawnguy  Offline OP
Joined: Aug 2014
Posts: 326
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.

Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666172
02/14/18 08:41 PM
02/14/18 08:41 PM
Joined: Jul 2014
Posts: 533
Wisconsin
Srt20 Offline
Srt20  Offline
Joined: Jul 2014
Posts: 533
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.

Last edited by Srt20; 02/14/18 08:42 PM.
Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666174
02/14/18 08:46 PM
02/14/18 08:46 PM
Joined: Dec 2017
Posts: 1,031
US
JLTD Offline
JLTD  Offline
Joined: Dec 2017
Posts: 1,031
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

Last edited by JLTD; 02/14/18 08:49 PM.

I'm a thickie; but a 20 has its place.

I use AMSOIL

Hers: 2008 Jeep Liberty 138k, SS 5w30/Amsoil

His: 2015 4Runner 41k, OE 5w20/Wix

Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666187
02/14/18 08:58 PM
02/14/18 08:58 PM
Joined: Apr 2016
Posts: 1,185
Campbellsville, KY
Alex_V Offline
Alex_V  Offline
Joined: Apr 2016
Posts: 1,185
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.


I like Chevy and Valvoline.

'13 Cruze, 1.4T, 113K
'85 GMC C3500, 454, 130K
'82 Mercedes 300CD, OM617 turbo, 169K
'67 Suburban, 350, 331K
Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666189
02/14/18 08:59 PM
02/14/18 08:59 PM
Joined: Oct 2009
Posts: 3,205
Wet side WA
JohnnyJohnson Online content
JohnnyJohnson  Online Content
Joined: Oct 2009
Posts: 3,205
Wet side WA
3.73 my guess is about a 10% loss in fuel mileage.


2004 Corolla 124642
Out: VML 5w-30 TG4967 OCI 5007 Miles
In: EDGE EP 5W-30 Bosch 3311 122537 7-18-18
2006 Duramax 74277
Out: T6 5W-40 M1-303
In: T6 5W-40 XG9100 74705 4-22-18
Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666197
02/14/18 09:03 PM
02/14/18 09:03 PM
Joined: Dec 2013
Posts: 2,170
Fort Worth, Texas
clinebarger Offline
clinebarger  Offline
Joined: Dec 2013
Posts: 2,170
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.


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/Auto
Re: 3.55 vs 3.73 ? [Re: JohnnyJohnson] #4666198
02/14/18 09:04 PM
02/14/18 09:04 PM
Joined: Aug 2003
Posts: 12,892
NE,Ohio
Rand Offline
Rand  Offline
Joined: Aug 2003
Posts: 12,892
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.


2019 Jeep Cherokee Trailhawk 2.0T
Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666204
02/14/18 09:12 PM
02/14/18 09:12 PM
Joined: Jun 2003
Posts: 33,070
ME
eljefino Offline
eljefino  Offline
Joined: Jun 2003
Posts: 33,070
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.

Re: 3.55 vs 3.73 ? [Re: clinebarger] #4666208
02/14/18 09:15 PM
02/14/18 09:15 PM
Joined: Apr 2008
Posts: 37,091
Ontario, Canada
OVERKILL Offline
OVERKILL  Offline
Joined: Apr 2008
Posts: 37,091
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.


2018 RAM 1500 Big Horn EcoDiesel
2016 Grand Cherokee SRT
Re: 3.55 vs 3.73 ? [Re: lawnguy] #4666224
02/14/18 09:31 PM
02/14/18 09:31 PM
Joined: Aug 2011
Posts: 2,021
Lakeside, CA
Blkstanger Offline
Blkstanger  Offline
Joined: Aug 2011
Posts: 2,021
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.


99 Jeep WJ 4.7 190,000 Maxlife 10w30, Fram Ultra.
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