3.55 vs 3.73 ?

Status
Not open for further replies.

DR1

Joined
Aug 17, 2014
Messages
749
Location
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?
 
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
 
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
 
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.gif


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:
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.
 
I currently have 245/70R16 106T Tires,and 16.8 custom wheels. I like a lower stance tan a tall truck. Thats just me.
 
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:
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:
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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
i have a 01 dakota. had a 3.55 gear. was a real dog in town. i had a 4.10 POSI put in. my in town MPG went from 8-9 to 12-13 in town . cause the trans would go into O.D. at 30 MPH. with the tach at 1400 RPM. i dont drive on the hi way.
 
Let's try this. A 3.00 axle would be a hwy cruiser,not so good on hills but would give good mpg. A 4.00 ratio would be screaming on the hwy,suck gas but would pull a hill well. Going from 3.55 to 3.73 would not be much of a change.
 
My 1997 F150 4.6 with 265-70-17 tires originally had 3.55 gears. I went to a 4.10 and at 65 MPH, I'm at 2050 RPM. Since I have a 1997, I had to change the speedo gear from a 17 tooth to a 21 tooth purchased at the Ford dealer. You can take this route, but since you have a PATS odometer, you can do a program change as well. My city mileage went up, highway mileage stays the same as long as I keep the speed limit. Towing and hill driving is way better. I found a 9.75" ring gear rear axle when I went to a 4.10 so I can have a bigger pinion gear. A 4.10 pinion gear is really small on an 8.8" ring gear. My truck had 120,000 miles when I did the change. I now have 299,000 miles on the truck. I will never go back to a tall (numerically low) gear. U joints last longer too because even though they spin faster, there is less torsional force on the driveshaft. Assuming your truck has 255-70-16 tires, going up to a 265-75-16 will give you 2050 RPM at 65 MPH. There are tons of ratio calculators online you can use.
 
Status
Not open for further replies.
Back
Top