Let’s talk Rear Axle - 4.7 vs 4.46

[Calicorks]

Hey guys, why would I want the 4.7 rear axel over the 4.46? What benefits dose the 4.46 have over the 4.7? Any And all thoughts are welcome. I’d like to hear benefits and downsides to both axel sizes.

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[Beachin 74]

4.7 would have more "mechanical advantage".

 

[Ipe]

Do you live where there's large hills or mountains? Do you ever plan on going to 35's? Do you ever plan on towing a trailer or loading your Bronco heavy? If you said YES to any of these things then you want the 4.7

Downside is that at highway speed you turn a few hundred RPM higher.

 

[Calicorks]

4.7 would have more "mechanical advantage".

Explain what you mean by more “Mechanical advantage”. I’m trying to decide if I want to spent the extra $$.

 

[Calicorks]

Do you live where there's large hills or mountains? Do you ever plan on going to 35's? Do you ever plan on towing a trailer or loading your Bronco heavy? If you said YES to any of these things then you want the 4.7

Downside is that at highway speed you turn a few hundred RPM higher.

This is helpful!

 

[EvlNvrDys]

4.7 would have more "mechanical advantage".

Explain what you mean by more “Mechanical advantage”. I’m trying to decide if I want to spent the extra $$.

I'm going to guess what @Beachin 74 is saying before he posts, it's a choice between Fuel Economy vs Performance.

Lower gears will turn larger tires easier. Lower gears will tow better than higher ones. Lower gears will accelerate quicker.

Higher gears = better fuel economy.

Mechanical Advantage ?

Edit: Now I'll let the technical people explain it better.

Edit #2: It's safe to say lower gears = lower end, higher gears = top end. Where do you want more performance?

 

[Austin26]

Those numbers aren't axle sizes. They're the gear ratios. It is just the torque multiplier. Like someone said, the higher number will cause more torque and acceleration but cause you to ride at highway speed at a higher rpm, say 100-200 rpms higher.

 

[Beachin 74]

Explain what you mean by more “Mechanical advantage”. I’m trying to decide if I want to spent the extra $$.

The ratio is how many times the driveshaft turns for 1 revolution of the wheel/tire. It would be easier on the drivetrain with the 4.7 than the 4.46 by just a little bit.
They are both decent ratios for an off roader.
Ipe explains it pretty good in post #3.

 

[Calicorks]

This is definitely helpful.

 

[Blksn955.o]

The 4.7 will have a quicker acceleration but will limit the top speed compared to say 3.73 gears. Also the higher numerically gear will offset the larger tire as the larger tire will travel more as it makes one rotation. This is why people that move to larger tires for off road often regear if they have a 3.73 or 4.11 as they will have a slower acceleration and the rpm drop in relation to mph moves the engine out of its sweet spot/intended rpm range. There are graphs online that show engine rpm, gear, and tire size that target stock and performance ranges that are very helpful

 

[Hey19]

If you’ve spent any time riding a multi geared bicycle, then the effect of gear ratio has manifested itself to you. 1st gear has you pedaling rapidly for a slow speed where 10th gear is a slower pedal cadence but a higher speed traveled.

 

[Nickp]

There is a bit of misinformation in this thread. I don’t think the 4.7 axle gearing will *necessarily* change the fuel economy number people care about; highway MPG’s. The 10th gear on the auto trans is so tall, my guess is that you’ll only ever been in 10th gear if you are on perfectly flat highway with the 4.7 gearing. With the lower gears, especially the 3.73, and the 32” tires, I would bet 10th gear will be basically unused. (It sounds like the ranger does use 10th gear pretty handily based on what I’ve read, but the ranger runs 255/65r/17’s which are a decent amount smaller and lighter than the 265/70r17’s that are on the 3.73 equipped Broncos. Base notwithstanding)

Where the 4.7 is going to be killer is in city driving since it will accelerate so much faster. YMMV.

 

[Austin26]

There is a bit of misinformation in this thread. I don’t think the 4.7 axle gearing will *necessarily* change the fuel economy number people care about; highway MPG’s. The 10th gear on the auto trans is so tall, my guess is that you’ll only ever been in 10th gear if you are on perfectly flat highway with the 4.7 gearing. With the lower gears, especially the 3.73, and the 32” tires, I would bet 10th gear will be basically unused. (It sounds like the ranger does use 10th gear pretty handily based on what I’ve read, but the ranger runs 255/65r/17’s which are a decent amount smaller and lighter than the 265/70r17’s that are on the 3.73 equipped Broncos. Base notwithstanding)

Where the 4.7 is going to be killer is in city driving since it will accelerate so much faster. YMMV.

I'll start by saying that I have no experience with this Ford 10-spd auto or the new manual transmission, but from looking at the overdrive on them, they're nearly the same at around 0.65. I'm inclined to think that it will indeed be used at speeds above 65 mph and that'll affect gas mileage some, though not more than a 100-150 rpm difference from 3.73s to 4.27s, maybe 200 rpms for 4.7. I'm guessing here but from a background with car gearing and older 2000s) Ford transmissions. Hell, if someone up sized their wheel diameter with new wheels and tires it could negate the gearing anyways but I'd have to do the math on that.

I did do math on rpm change for difference in gearing but not factoring air resistance which should play a bigger role at highway speeds. It was, like I said, only about a 150 higher depending on the wheel,dimater size (based on trims)

 

[Hey19]

In my F150, gas mileage is better at 18.6 mpg @ 1900 rpm and 75mph compared to about 17 mpg cruising at 85 mph which is around 2100 rpm. That tells me that a difference in gear ratio for a constant speed would have the same effect. Lower gears = higher engine speed = lower mpg. That difference for me is a wash, but it is a difference.

 

[BroncoT]

I did the math in a different thread borrowing RPM from yet another thread (the forum needs an FAQ because searching is difficult with so many overlapping threads and endless pages of comments).

There is a 123 RPM difference at 70 MPH assuming both are using 10th gear. Nickp correctly pointed out that 0.65 overdrive is STEEP and many models will thus cruise in 9th. Maybe the 4.46 will too, but it will hit 10th eventually (65? 70? 75?). I’m talking BL 33” tires only since that’s really the only tough decision I’m aware of.

Wherever that happens and using my rule of thumb where 100 RMP lower equates to 1% fuel economy savings off baseline (based on diesel where I have seen data to back this up), we have 1.23% worse fuel economy on highway. Assume 25 MPG at 70 MPH (the only time this matters unless you’re punching it a ton off the line).

25 - (25 * 0.0123) = 24.7 MPH. Not much difference.

Honestly, the difference will be minor all around as the two ratios are pretty close. I’m basing the decision on this:

1. Whine. Do I think that extra 123 RPM will be noisy? I doubt it on the 2.7L but maybe on the 2.3L.

2. Resale and Upgrade. Even if I never put on 35” tires (I’m not planning to), I think having 4.7s could help resale value. Lots of people will like 35s and I’ll have perfect gearing for their upgrade.

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