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Gearing and tire size

Oorko

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In reading through the forums I've seen a few comments about the 3.73 gearing and stock tires (in my case P255/75R17) that sound like if you go with 4.27 that you'd actually be better off with slightly larger tires for power, MPG, etc. Is that correct? When I eventually get my Bronco my original plan was to switch out the 255/75R17 tires for 285/70R17 and stick with 3.73. Having the locker would obviously be handy when off-roading but from a gearing standpoint would it make much difference in MPG if I went with 4.27 and the larger 285 tires? Thanks!
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RagnarKon

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  • The higher the gearing, the more torque ("get up and go") the vehicle will have, at the expense of top-end speed and MPG
  • The lower the gearing, the less torque a vehicle will have, but it will have a higher top-end speed and higher MPG
So... the real question is if you need the extra torque or not. And that ultimately depends on what you are doing with the vehicle. The larger the tire the more torque is needed to move the tire. But if you are spending 99% of your time on the pavement, having that extra torque isn't that important.

Take 37" tires for example. If you are off-roading extremely difficult trails, then you'll want the 4.70 axle ratio. If you are on more moderate off-road trails, the 4.46 axle ratio will probably be fine. If you are pavement and will rarely go off-road, you probably only really need the 4.27 axle ratio. The engine you get also plays a factor. The 2.7L has more torque than the 2.3L, so you can get away with a lower gear ratio on the 2.7L engine than you could with the 2.3L engine.

So... I guess my question is, what do you intend on doing with your Bronco??

---

p.s. I built a calculator that will hopefully demonstrate all of this a bit more. (Although if this is a new concept for you, it probably won't be all that helpful.)

Bronco Engine RPM Calculator (for specific tire sizes & axle ratios)
 

kodiakisland

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The MPG of a rolling brick is going to be bad either way. Go for performance, which is lower gears. No one ever wishes they had higher gears in a 4WD.
 

goatman2

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Based on many many years of Jeep and off road experience, it is pretty universal to get better mileage with the gear ratio that matches the tire size. I've seen many cases of adding large tires, mileage drops, then change gears and mileage comes back up, obviously not to stock levels but better than with stock gearing and large tires. For power, shift points, trans TCM management, mileage, and low range gearing for off road, the gear ratio that matches the tires is superior.
 

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Bradley Thornton

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dgorsett

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I have a bb with 4.27 Locker , 2.3, auto and get 23 mpg. Some times lower gears get better mileage not to mention better power and locker to boot. Go with the 4.27,you won't regret it and would be set for bigger tires.
 
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Oorko

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Thanks for the feedback. In all honesty It'll probably only be going off road a very small percentage of the time. So looking at that great calculator and the chart, If I upgraded to 33's and stuck with 3.73 it may give better mpg but would be a decent bit slower (I'm going with the 6 instead of the 4 so maybe the hit wouldn't be as noticeable?) but going with the lockers/4.27 option would put me right in the sweet spot of power/performance for 33 inch tires and of course I'd have better off road performance.
 

da_jokker

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Yeah your bad gas mileage doesn't come from cruising, it comes from accelerating (In our case being under boost). That is why city mileage is usually suckier than highway because of all the "go" parts of stop and go.

So having high gears with large tires make sure engine work harder to get up to speed so the less time you need to get it to speed, the better fuel mileage (again as long as you're not straining the engine at freeway speeds and staying under boost)
 

Techun

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The higher the gearing, the more torque ("get up and go") the vehicle will have, at the expense of top-end speed and MPG
Sentence above only really applies when you're driving in the highest and lowest gears of the transmission.
 

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RagnarKon

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Sentence above only really applies when you're driving in the highest and lowest gears of the transmission.
True.

Since the Bronco has a 10-speed automatic, the top-end speed is basically a non-issue. Not that it ever was really an issue to begin with... it's not a race car.

The lowest gear does matter for an off-roading vehicle, but the transfer case gearing alone will solve that issue for 95% of the folks out there.
 

goatman2

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Thanks for the feedback. In all honesty It'll probably only be going off road a very small percentage of the time. So looking at that great calculator and the chart, If I upgraded to 33's and stuck with 3.73 it may give better mpg but would be a decent bit slower (I'm going with the 6 instead of the 4 so maybe the hit wouldn't be as noticeable?) but going with the lockers/4.27 option would put me right in the sweet spot of power/performance for 33 inch tires and of course I'd have better off road performance.
It may not give better mileage, and most likely won't give better mileage to have 33's and the 3.73 ratio. A lower rpm doesn't guarantee better mileage since mileage is also based on load, meaning throttle pressure. In the real world, especially with a 10 spd transmission, the trans will just go to a lower gear with the load and throttle input with the 33's and 3.73 gearing. To assume you would get better mileage you would have to assume that you will always be driving on perfectly flat ground with no wind, and even that is no guarantee. Better mileage is matching the tire to the ratio, quite a bit of experience and data to show that.
 

dgorsett

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I have a bb with 4.27 Locker , 2.3, auto and get 23 mpg. Some times lower gears get better mileage not to mention better power and locker to boot. Go with the 4.27,you won't regret it and would be set for bigger tires.
BTW. I generally drive the speed limit. This maximizes mileage not only due to wind resistance of the Bronco brick, but also minimizes coming up on some slow old fart (me), decelerating, then accelerating to pass, in and out of the throttle kills mileage.
 

Lukesky1

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I have a bb with 4.27 Locker , 2.3, auto and get 23 mpg. Some times lower gears get better mileage not to mention better power and locker to boot. Go with the 4.27,you won't regret it and would be set for bigger tires.
I’m looking at the same build, is the 4.27 enough for 37’s?

I don’t see the point in the Sasquatch if I’m going to replace tires, coil overs,uca’s
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