Let’s talk Rear Axle - 4.7 vs 4.46

[Spooled]

so regardless of transmission and engine choice, is it safe to assume the Bronco will be a gas hog? what's the estimated average MPG? similar to the Ranger or worse?

Also, would the 4.27 be adequate for 33's? i'm thinking about the least strain under normal driving conditions (city/hwy)

Depends on your definition of a gas hog.

4.27 is plenty for 33's.

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[wjfawb0 [hacked account]]

First off, this thread title has bothered me for a while. The bronco is 4wd, so this concerns both the front and rear axle ratios which have to match or you will have issues when you shift into 4wd if not before that.

Drag on a car varies with the square of the speed.

https://ecomodder.com/wiki/Road_Load_Equation
https://auto.howstuffworks.com/question497.htm
http://www.virtual-car.org/wheels/hybrid_road_load_model.html

In my experience crawling a manual transmission wrangler with a 4:1 transfer case and 4.10:1 axle gears, fuel injection can make it very touchy. It can get so bad that it's difficult to smoothly modulate the gas while offroading at say 3mph while crawling. That's why it's usually easier just to let it idle and crawl. Maybe ford will do something about this in the bronco to smooth out the throttle input while crawling. I know it took a lot of practice in my 2015 wrangler rubicon to keep from bouncing when doing rough offroad in crawling gears.

Most wrangler folks prefer going to 4.88 or 5.13 gears when they go 35" or larger tires.

Automatics are totally different due to the slipping that happens all the time. If you don't offroad much, having the right gears for the tires will not matter as much, but you will accelerate faster with lower gears.

More info on gears and carriers with pics: https://www.crawlpedia.com/thick_gears.htm

 

[Felix808]

The 4.7's will pay dividends when off road dancing in rocks or other technical areas of a trail by allowing you to travel slower & maintain better control. Hitting obstacles at speed is not the preferred method as it increases the chances of incurring damage to the vehicle. The lower gears allows the vehicle to work / strain less in these situations so you don't need as much skinny pedal while also allowing better engine braking on descents. With the 10-speed trans, you have plenty of headroom for higher speeds so having lower diff gears will not be as much a factor on the hi-way.

 

[Felix808]

In my experience crawling a manual transmission wrangler with a 4:1 transfer case and 4.10:1 axle gears, fuel injection can make it very touchy. It can get so bad that it's difficult to smoothly modulate the gas while offroading at say 3mph while crawling. That's why it's usually easier just to let it idle and crawl.
More info on gears and carriers with pics: https://www.crawlpedia.com/thick_gears.htm

Installing a hand throttle in my XJ was the best mod outside of my Atlas 5.0 which put me at 80:1. It's hard to be smooth in a manual when your foot is bouncing

 

[Karl_in_Chicago]

so regardless of transmission and engine choice, is it safe to assume the Bronco will be a gas hog? what's the estimated average MPG? similar to the Ranger or worse?

Also, would the 4.27 be adequate for 33's? i'm thinking about the least strain under normal driving conditions (city/hwy)

Well, depending on chosen options it has an estimated curb weight that could exceed two and a half tons, has worse aerodynamics than a 70's Volvo (the "brick") and oversize tires. So it really depends on one's definition of "gas hog" and what it is being compared to. I think most people, when evaluating it against all motor vehicles, would think it lands in the gas hog class. When comparing it to other, similar, vehicles enthusiasts might find it competitive. Since we don't yet have the EPA estimates it's too early to say but I'm guessing that (again, depending on engine, tire and gearing choices) we might find the mileage figures pleasantly surprising. In general, though, people buying vehicles like these have already put MPG relatively low on their list of "must have". Like any vehicle mileage also will vary greatly with how the vehicle is operated. For every person driving around a VW GTI and boasting they are exceeding the EPA 32 mpg highway rating there's somebody else driving the same car and barely getting 20.

 

[Drex]

Drag on a car varies with the square of the speed.

Yep, Drag is a squared function, but the question was related as you how much power was needed to propel a vehicle at a specific velocity. That is a cubic function as there is another velocity factor.
Start with the definition of pressure as force per area. Solve it for force.

P =	F	⇒	F = PA
place the generic symbol F for force with the more specific symbol R for drag. (You could also use D if you wanted to.) Drop in Bernoulli's equation for the pressure in a moving fluid…

R = PA =	⎛ ⎜ ⎝	1	ρv2	⎞ ⎟ ⎠	A
2

R = ½ρCAv*v

Which is drag formula in a fluid. v is velocity and the other terms are constant so they will drop out in a comparison of velocity change. It is steady state. Not doing anything but counteracting drag, you still need more power to actually move the vehicle and maintain velocity.
Obviously from beginner physics; power equals force x velocity. Substitute the drag formula for force you need to overcome.

Power equals (½ρCAv^2) x (v)elocity. Simplifying to Power equals ½ρCAv^3. To compare power required to change velocities for the same object, it becomes a simple ratio where everything cancels out except v^3. As an example if it takes 25 horsepower to continually move a car at 70MPH, to hold 90MPH would take 90^3/70^3 multiplied by 25 horsepower. Which is 53 horses (you need a bunch more to accelerate to that speed as your rate of acceleration would drop to zero by the time your got to 90MPH.

Short version;. Air resistance is a squared function of velocity, the power needed to move a vehicle at that velocity is a cubic function of velocity.

 

[MannyG20]

Yep, Drag is a squared function, but the question was related as you how much power was needed to propel a vehicle at a specific velocity. That is a cubic function as there is another velocity factor.
Start with the definition of pressure as force per area. Solve it for force.

P =	F	⇒	F = PA
place the generic symbol F for force with the more specific symbol R for drag. (You could also use D if you wanted to.) Drop in Bernoulli's equation for the pressure in a moving fluid…

R = PA =	⎛ ⎜ ⎝	1	ρv2	⎞ ⎟ ⎠	A
2

R = ½ρCAv*v

Which is drag formula in a fluid. v is velocity and the other terms are constant so they will drop out in a comparison of velocity change. It is steady state. Not doing anything but counteracting drag, you still need more power to actually move the vehicle and maintain velocity.
Obviously from beginner physics; power equals force x velocity. Substitute the drag formula for force you need to overcome.

Power equals (½ρCAv^2) x (v)elocity. Simplifying to Power equals ½ρCAv^3. To compare power required to change velocities for the same object, it becomes a simple ratio where everything cancels out except v^3. As an example if it takes 25 horsepower to continually move a car at 70MPH, to hold 90MPH would take 90^3/70^3 multiplied by 25 horsepower. Which is 53 horses (you need a bunch more to accelerate to that speed as your rate of acceleration would drop to zero by the time your got to 90MPH.

Short version;. Air resistance is a squared function of velocity, the power needed to move a vehicle at that velocity is a cubic function of velocity.

this is exactly what I was going to say. Good job Drex!!

 

[wjfawb0 [hacked account]]

You are correct on power versus force. I should have read more carefully. Thanks for the clarification.

 

[Charky]

And now for a decidedly un-engineered (yea, just made that up) and politically incorrect response; just go with the lower gear, punch it every now and then, have fun, and pay the extra $200-$300 a year in lost mpg.

 

[BWS_TX]

If I understand this all correctly as long as I don't might a little more money in gas and would like better acceleration then I should go with the 4.7 on the BL 33". Correct?

 

[Bdockins]

If I understand this all correctly as long as I don't might a little more money in gas and would like better acceleration then I should go with the 4.7 on the BL 33". Correct?

ALL DAMN DAY

 

[Khayze427]

I don't know if this is a glitch or not, but on the B&P I can apparently select 4.7 ratio on the BL (auto 2.7L) without doing a SAS? Anyone know if this is legit or not?

I don’t know but I hope it is true.

 

[Rydfree]

Hitting obstacles at speed is not the preferred method as it increases the chances of incurring damage to the vehicle.

 

[Felix808]

Boy that channel is annoying but yeah that intro kind of shows what I'm talking about.

 

[mrcx]

My 90 Bronco got 8mpg. If I can get 19-22 combined I will pee myself in excitement.

My current car. '67 Camaro, probably gets 8 mpg. Before that I had a Wrangler and before that a '53 Chevy 5300. The Bronco might get me the highest MPG I've had in the last 10-15 years.

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