Will the 4.7 final drive ratio kill MPG and TOP end speed for use as practical daily driver?

AzScorpion

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I’m not sure how useful this chart is. You need to know the final drive ratio, including transmission gearing. The 10A has quite the spread on gears.
This is correct and I was going to say even trying to speculate the mpg to the Jeep is not going to be accurate. I can be driving 50 mph on a open surface street and in 9th gear with this tranny. It's so much more spread out than than even the old 8 speed ones.





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AMK610

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Higher final drive ratio = higher rpm = worse mpg.
Bigger heavier tires = more throttle to get it moving and keep it moving = worse mpg
The higher gearing helps keep the peppiness when you pair it with bigger heavier tires. But in order to keep that feeling, it’s going to be less economical. None of those changes will ever increase economy, just help the feel and longevity of your drivetrain
 

Used2jeep

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That's a good thought. Was planning on staying with 33's. Are you saying if I went with 35's my mpg would be better but I'd just be slower off the line?
I would think that it would be similar. Going bigger will absolutely make your vehicle "handle more trucklike" but with the almost offsetting gears/tire size then you shouldn't notice a HUGE loss. * "Huge" is different for each person.
 

Bdockins

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I was wondering this myself.

What if sasquatch on all trims is a one time gimmick and will be phased out if they discover that reducing access to packages on certain trims increases profits and manufacturing efficiency.

I don't need sasquatch but I'm getting a basesquach in part because of this very reason.
Yes so I could see a base Squatch as a sort of special trim. It is between that and a bare Badlands for me.
 

dejones64

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Someone posted this link out here somewhere, it's an rpm calculator given your gears, tire size and speed.

https://spicerparts.com/calculators/engine-rpm-calculator

Edit: In a manual, when you shift makes a difference in mileage also. I heard somewhere shifting at about 2200-2500 rpm is optimal for mileage.
 

Drex

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Yea but when you add bigger tires doesnt your MPG drop lower also?

Maybe im not understanding something here, in my experience changing to a higher number gear 4.10 or 4.7 hurts your fuel economy, also the bigger your tires are 33 to 35inch hurts your fuel economy so why together would it be a wash? I'm not a mechanic but my simple logic would tell me together they would make your fuel economy even worse not wash each other out. But please let me know if my understanding is wrong.
The gearing to which you are referring is at the differential, so that the driveshaft coming into it spins (in your example; either 4.1 or 4.7 times for each revolution of the wheel, which means one revolution of the tire as well. A taller tire has a greater circumference so for each revolution the vehicle travels further. (this is the key to the whole understanding. Taller tire = more travel per revolution, which means traveling further per RPM of the engine.)

Real World example using a Badlands; The 4.46 gears are about 5% taller than the 4.70 gears. (4.70/4.46 = 1.053) difference, so the same size tire on a 4.70 (after running through the other gears) will turn 1.053 times more than with the 4.46 gearing (yes, a small 5.3% change. )

The 315/70-17 (35's) tires turn out to have about 5% larger circumference than the 285 tires on a Badlands (use a tire calculator online if you are not familiar with the calculations). Since the 35's go about 5% further on each revolution and if you put a 35" tire with the the 4.70 gears(which spin 5.3% less for the same input), it travels almost exactly the same distance as the 33" tire with the 4.46 gearing for each engine RPM.

That means that the RPM for the same speed are going to be almost exactly the same, a wash. Also since the power is put to the ground through the tire, both scenarios have equal mechanical advantage (assuming the gearing of the transmission is the same for both).

As it will take more energy to turn a bigger, heavier, larger frontal area, M/T tread 35" tire than the 33" A/T tire with the same effective gearing at the contact point of the road/terrain and the rubber, you get worse acceleration, worse fuel economy, and so on. So you will get worse fuel economy with the bigger tires, but the effective gearing at the tire/ground intersection is roughly the same. An effective gearing wash, but you take an economy hit anyway.

Hopefully I have not made it worse.
 

makatron

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Going from 33 to 35 will likely add 10-15 pounds per tyre, depending how agressive the thread is. So yeah, the 4.7:1 ratio makes sense when you think you need the power to spin all that mass whilst going uphill in the trails.
 

Emcfetr1

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Not reading all 4 pages but do the math I have many times on many sights . Grand scheme of your gas mileage goes from 20 to 16 it’s literally not that much money per year to care . If you do 15k miles a year there only $350 ish more a year in gas to have badass 35’s (16 mpg) instead of 32’s (20 mpg) Well worth it
 

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Going from 33 to 35 will likely add 10-15 pounds per tyre, depending how agressive the thread is. So yeah, the 4.7:1 ratio makes sense when you think you need the power to spin all that mass whilst going uphill in the trails.
The change in diameter has much more effect than the change in weight. The change in diameter affects the operating RPM of the engine at a given speed. To get "factory performance" when you change from one tire size to another you do some simple arithmetic to choose your ratio.

Old gear ratio * new tire size / old tire size = new gear ratio.

The added weight and rolling resistance of the tire does affect performance. But it's only the reason you would always round up. The biggest factor is the change in diameter.


To generally address the topic:

Every engine has an optimal operating range where it performs best. Getting the engine to cruise at that RPM will give you the best balance of power and mileage. Obviously, if you go way higher in RPM your mileage will drop off. But what's less obvious is that if you drop RPMs too low you will also lose mileage. The engine is out of it's power band and it struggles to make power. So you really need to know the optimal operating range of the engine and match the gears to the tire size to get the best performance and mileage. If your gears are too short (higher numbers) you can compromise highway mileage. If your gears are too tall (lower numbers) you will struggle to maintain speed in the top overdrive and you will lose mileage. Of course, you can compensate by down shifting the transmission but why have a setup that's not taking full advantage of your overdrive?

As an example:

I had a 2012 Wrangler with a 6 speed manual transmission. I was running the stock 32" tall tires and I had 3.21 gears in the axle from the factory. I did an experiment where I ran a 100 mile loop in 6th gear (top gear) and then ran the same loop in 5th gear. Turns out I got slightly better mileage in 5th gear and I had enough power to hold speed going up the hills. The difference was that 6th gear put my RPMs too low and the engine (3.6 Pentastar V6) didn't make good power at the lower RPMs. But in 5th, I got my RPMs up above 2,000 and the engine was much happier. The optional 3.73 gears in that Wrangler are much more appropriate for 32" tires. And the 4.10 gears in the Rubicon version with the same diameter tires are even better. It's why I went all the way to 4.56 when I upgraded to 35" tires, and still wish I would have gone with 4.88's.



Every vehicle is a system with many parts. The axle gears is just one number in a big equation. All of the rest is important to take into consideration when choosing what axle ratio you want. Long term plans for the vehicle are important as well. Are you going to buy your Bronco with stock small tires but immediately put bigger tires on? Get the shorter (higher number) gears for the tires you are planning on running. Are you going to daily drive your Bronco and never change the tires? Maybe get the taller (lower number) gears. Are you going to rock crawl and stick with stock smaller tires? Maybe shorter (higher number) gears are for you.

First figure out exactly what you're going to do. Then use that information to pick your gears.
 

Hkak45

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Difference in your mustang is you only changed the gearing.

They are talking about both a tire change and gearing. When you combine the two it's almost a wash.
This.... I changed my truck gears to 4.56 and threw on some 35’s with a custom tune and my truck now gets 1mpg better than it did before....... Now I am happily at 13MPG😂
the gear ratio changes with the bigger tire sizes so Ford can get roughly the same MPG as the other trims. With that said, yes top end will be less but acceleration will be quicker lol
 

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Left side is 4.7 rear axle ratio with LT315/70R17 tires. Right side is 4.7 rear axle ratio with LT285/70R17 tires. Chart is automatic transmission gear vs speed (I picked 3 speeds) to give engine RPM. "Aint nobody" going to be worried about being stuck at high rpms with 3 overdrive gears!

2020-11-23 (8).png


Left side is 4.7 axle ratio with LT315/70R17 tires. Right side is 4.46 axle ratio with LT285/70R17 tires.

2020-11-23 (7).png
 
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mneblett

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In a Badlands I expect
You would lose at least 1 MPG from the wider, more aggressive tread M/T tires on a Sasquatch package.
In my Badlands, I expect to lose 10 mpg with my wife's right foot.
 

Used2jeep

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