Rear Axle Drive Ratio Options

I would not opt for 4.7 gearing for mud and snow. The gear ratio is a torque multiplier, so you are multiplying torque by 3.73 or 4.7. You will have approximately 27% more torque with the 4.7, but that doesn't help in mud/snow. It helps in rocks when you want gear reduction and climbing, but not when you want to create wheelspin to clear mud/snow from tires and maintain momentum. It is also a multiplier that you see in RPM for any given speed, so you are turning faster, generating more revolutions per minute/mile. Your engine is going bang more often with 4.7 than with 3.73 for any given speed. So, unless that additional torque is necessary to move something, the vehicle, with or without a trailer, etc. you are turning and burning more fuel for a given speed. It's all a trade-off, but based solely on your question, mud/snow versus rock and your concern about fuel economy, I think 3.73 is a better choice.

With an auto you can go with the lower ratio. On road you want the 3.73 for gas mileage. With hi and lo in the transfer case you will have gear ratios appropriate for what you want to do.

Thanks! I suspected this was true, but now I know.... I'll focus on the auto with the 3.73.

Are you getting oversized tires? If you go oversized tires, you will lose torque and may want to drop to lower the rear if that comes into play...

Good point, the tires are the last and largest gear in the system, from the transmission to t-case to differentials, they then have to turn the next gear in the chain which is the tires. What's nice though is the tires are the largest and therefore the least impactful (in some ways) to the system. A change from 31" to 35" tires is only a 12% change (granted you have a small weight difference too). The gearing difference between 3.73 and 4.7 is much more significant, at 27%, and the torque converter of the auto trans makes up for the 12% difference for most people in most situations. First gear in the auto is lower than first in the manual (not considering the "granny" first, it's 10% lower than the manual and offers a lot more gearing options). If you really think you'll jump from 31" to 35" tires (as an example) you're probably better off getting a 4.27 and auto if that is a combination; however, the auto is going to give you a low first gear, off road or off the line. That all being said, I think the auto, with 3.73 or 4.27, are going to serve a person well that is more concerned about economy and snow/mud.

It's looking like I'll go with the Outer Banks and the 32s. Hate the OB "not a grille" grille though. Hoping it's true that I'll be able to swap it out for the Badlands one.

Jolly Rancher

It's looking like I'll go with the Outer Banks and the 32s. Hate the OB "not a grille" grille though. Hoping it's true that I'll be able to swap it out for the Badlands one.

The parts dept should be able to get grills, I plan on getting another one and painting it white for my Big Bend..

See the following tables for all your questions about how the various gear and tire combinations will affect crawl speed and highway speed.

AcesandEights

Good point, the tires are the last and largest gear in the system, from the transmission to t-case to differentials, they then have to turn the next gear in the chain which is the tires. What's nice though is the tires are the largest and therefore the least impactful (in some ways) to the system. A change from 31" to 35" tires is only a 12% change (granted you have a small weight difference too). The gearing difference between 3.73 and 4.7 is much more significant, at 27%, and the torque converter of the auto trans makes up for the 12% difference for most people in most situations. First gear in the auto is lower than first in the manual (not considering the "granny" first, it's 10% lower than the manual and offers a lot more gearing options). If you really think you'll jump from 31" to 35" tires (as an example) you're probably better off getting a 4.27 and auto if that is a combination; however, the auto is going to give you a low first gear, off road or off the line. That all being said, I think the auto, with 3.73 or 4.27, are going to serve a person well that is more concerned about economy and snow/mud.

The tires are the "last gear" in theory. All those percentage changes you mentioned are RPM percentages seen on the gauge, not actually torque numbers. HP and Torque ratings on a given drive train are not dependent on tire size. You may feel the difference in performance, but a 6.69 first gear transmission with a 2.42 transfer case with 4.7 axles will still produce a 76:1 final drive ratio whether you've got 30" tires or 37" tires.

Edited to add: in brevity; that's like saying the transmission gearing doesn't matter because the engine hp/torque numbers are (whatever they are), but we know the transmission gearing matters. If we take it a step further, it would be like saying the engine and transmission are what matter, while leaving out the transfer case ratio. Of course that again is like leaving out the final drive ratio (differential gearing). Overall, the torque numbers are important, and the gearing throughout the system is important, including tire size. Ultimately what we're really talking about is drivability. Each component of that system plays a part in the overall drivability, including the tire size.



Crawl ratio is a different term to represent the same thing, at least in my opinion. I don't think we're saying anything contrary. However, the way the vehicle handles off road/on road conditions changes with tire size, even if the crawl ratio remains constant or consistent, because your load(ing) changes with tire size. Your final drive ratio isn't really the same as crawl ratio, although I know what you mean. Final drive is the differential ratio, but all of those terms leave out the last "gear" in the system, the tires. I think using crawl ratio is ending the conversation too soon, just like only using engine hp/torque numbers and not talking about the rest of the drivetrain. The engine produces a certain hp/torque rating, but that's not the end of the story. There is a transmission, t-case, differential...and, tire size. If you stop at crawl ratio, it doesn't give the person doing the research the whole story about drivability.

You could also use the tire size as a metaphor for load. The larger the tire, the heavier the trailer you're trying to get moving. Doesn't really matter the crawl ratio in that context, or maybe it does; ultimately to get the load moving you want torque, and the multipliers, which are also the multipliers for crawl ratio, are the gears in the system. The OP is concerned about economy and mud/snow; really concerned less about crawl ratio, but momentum and wheel spin, at least in my opinion. If the OP were more concerned, or the question was one of climbing up and over, I'd say the manual trans in granny-low, with 4.7 in the pumpkins is going to give you a better experience. The OP says he/she is less concerned about climbing, so I think an auto and higher (lower number) gearing is the way to go. Just my opinion of course.

Your final drive ratio, or crawl ratio, is the multiplier of torque that turns the gear we call the tires. Luckily the auto trans, the differentials, allow for different torque multipliers/crawl ratios, which applies to the OP questions about economy and use. Typically you want your RPM lower for better economy, but not always/necessarily, if the vehicle isn't running as efficiently for the load (and that can be tire size). If you have taller tires, as an example, you may want lower gearing all the time, because your vehicle may not be running in the most efficient RPM/torque range around town, or the highway commute, or whatever your driving conditions are most of the time. All of it is a trade-off, because we don't drive consistently all the time; we find ourselves on the highway some days, and in town driving other times. Gotta pick one that suits you the most, the most often.

Yep, I think auto with 3.73 is how I'm going to go. I do need to pull a very small trailer a few times a year (<1500 lbs), and I was originally assuming I'd want the bigger engine, just because I'm used to the V8, but I'm not convinced that's the case.

A related question...anyone know what the tow package will include? The Build & Price tool gives its cost as $595 and looks like just includes receiver hitch and wiring. If I get the Outer Banks, I'll have to pay extra for Aux switches.
I was hoping it would be at least comparable to the Jeep Unlt:
Jeep Unlimited Trailer Tow and HD Electrical Group $795
Auxiliary Switches
Class II Receiver Hitch
7– and 4–Pin Wiring Harness
240 Amp Alternator
700 Amp Maintenance Free Battery

I’m rounding the corner on finalizing my build and have a question regarding gear ratio and tires. I’ve settled on Base V6 with 3.73 gears. If I ever wanted to run 33’s, what are the gear components that would need to be changed/upgraded?

Guys, Ive never had large tire sizes, wondering if this combination is a doable thing: 4 cyl eco boost, auto trans, 33 or 35 tires, 3.73 final ration. I wont be doing rock crawling, but might do mud or sand in addition to the regular in town driving. Love the 35s for the looks lol Thanks!

I have run 33s before on 3.55s in a full size Bronc so can be done but was sluggish. Might not be the best for crawling but overall is doable. If you are going to run the 35s or think you will in the future then I would consider the shorter gear ratio options like the 4.46s or 4.70s. You could also run 35s but I think you would have a better overall with the correct gearing. Ford paired the 35s with the 4.70s for a reason, to maintain the overall balance and keep the Bronc in the desired power band for the best balance of performance and efficiency.