33's versus 35's

NCOBX

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Fantastic explanation! I have considered making this point in response to many of the posts regarding tire size and mpg but opted out because I couldn't articulate my explanation this well.
Several people are greatly exaggerating the size of 35s, and the fuel economy difference between properly geared 33s and 35s.

My truck came from factory with the same 315/70r17 size tires and 4.10 gearing. I added 37s and 4.56 gearing and did zero lift. I gained 2 MPG over my stock factory original truck by going to steeper gears with the bigger tires. My effective gearing is steeper than factory setup with original gears/tires.

The easier the engine works the better your fuel economy will be.
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Straight 6

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Several people are greatly exaggerating the size of 35s, and the fuel economy difference between properly geared 33s and 35s.

My truck came from factory with the same 315/70r17 size tires and 4.10 gearing. I added 37s and 4.56 gearing and did zero lift. I gained 2 MPG over my stock factory original truck by going to steeper gears with the bigger tires. My effective gearing is steeper than factory setup with original gears/tires.

The easier the engine works the better your fuel economy will be.
It's not mainly diameter that causes the mpg dip
It's the 255, 265, 285 mm wide instead of 315 wide

Wider tire is harder to roll and harder to push through the air.

30-60 mm of extra width per tire is enough to reduce efficiency

Think of how a road racing bike has skinny tires and a mountain bike has wide tires.
 

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I am in exactly the same boat. I would like all the extras that come with SQ packag
so which would be more negatively impactive...

1) Getting a Bronco with 33s and it's gearing, and down the road tossing on 35s
2) Getting a Bronco with 35s and it's gearing, and down the road dropping to 33s
I have the exact same question. I do not have the knowledge base to even a remotely guess.

For me it, is going to determine the whole build/trim level.
 

fergthulhu

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so which would be more negatively impactive...

1) Getting a Bronco with 33s and it's gearing, and down the road tossing on 35s
2) Getting a Bronco with 35s and it's gearing, and down the road dropping to 33s
I am in exactly the same boat. I would like all the extras that come with SQ packag

I have the exact same question. I do not have the knowledge base to even a remotely guess.

For me it, is going to determine the whole build/trim level.
Not an engineer, but there's a lot of factors that would go into which is better for you personally: driving style, specs of tires, etc. However generally speaking, the squatch going down to 33" tires will require higher RPMs to maintain the same speed because of the higher ratio (I think about 6% more based on generic tire circumference), also offering a slower minimum speed and increasing possible wheel torque. Conversely the BL going up to 35" would require about the same amount less RPMs to maintain the same speed as it did on 33s, which would also increase the minimum speed and decrease possible wheel torque. I'd suspect that these values would then have to take into account the tire characteristics like profile and rotational mass but I'd only be guessing at their effect at this point.

It appears they have made the ratios such that the 4.46 on 33s and the 4.7 on 35s end up performing mostly the same. So it's really just a trade-off depending on what your use case is.

Edit: Forgot to add...link to the EE video where he talks about ratios and their effect. He's talking about the manual and 2.3l, but you can plug in your own values if you want to see how it effects these things.

Bronco Gearing - Engineering Explained
 
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Straight 6

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so which would be more negatively impactive...

1) Getting a Bronco with 33s and it's gearing, and down the road tossing on 35s
2) Getting a Bronco with 35s and it's gearing, and down the road dropping to 33s

Same width on both options?
 

BroncoFanBoy

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Not an engineer, but there's a lot of factors that would go into which is better for you personally: driving style, specs of tires, etc. However generally speaking, the squatch going down to 33" tires will require higher RPMs to maintain the same speed because of the higher ratio (I think about 6% more based on generic tire circumference), also offering a slower minimum speed and increasing possible wheel torque. Conversely the BL going up to 35" would require about the same amount less RPMs to maintain the same speed as it did on 33s, which would also increase the minimum speed and decrease possible wheel torque. I'd suspect that these values would then have to take into account the tire characteristics like profile and rotational mass but I'd only be guessing at their effect at this point.

It appears they have made the ratios such that the 4.46 on 33s and the 4.7 on 35s end up performing mostly the same. So it's really just a trade-off depending on what your use case is.

Edit: Forgot to add...link to the EE video where he talks about ratios and their effect. He's talking about the manual and 2.3l, but you can plug in your own values if you want to see how it effects these things.

Bronco Gearing - Engineering Explained
I was thinking if you want the option to run 33s or 35s, choosing a trim such as Black diamond with 4.46 gearing would be a better option than Sasquatch’s 4.7. Thoughts?
 

da_jokker

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Along these lines, I just saw a video from trail recon where he did a non scientific test of MPG between before and after his gearing and tires on his gladiator. He gained 1 mpg after going with larger tires and lower gears (or should I say higher gear ratio)

The main point is he didn't take a hit and that is a big question on many minds.
 

fergthulhu

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I was thinking if you want the option to run 33s or 35s, choosing a trim such as Black diamond with 4.46 gearing would be a better option than Sasquatch’s 4.7. Thoughts?
That makes the most sense to me, since most of my travel will likely be on the road at highway speeds and I don't plan to spend a lot of time rock crawling. I think it's pretty minor either way though so I don't think you can go too wrong with these minor differences.
 

kodiakisland

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so which would be more negatively impactive...

1) Getting a Bronco with 33s and it's gearing, and down the road tossing on 35s
2) Getting a Bronco with 35s and it's gearing, and down the road dropping to 33s
Always get the gears. Rarely does anyone wish for higher gears. Unless your Bronco is going to spend the vast majority of the time at 80mph+ or you’re going to run 31in tires, get the 4.70s.
 
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da_jokker

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Always get the gears. Rarely does anyone wish for higher gears. Unless your Bronco is going to spend the vast majority of the time at 80mph+ or you’re going to run 31in tires, get the 4.70s.
Thanks.. that was the missing part.. I tend to drive reasonable on trips, and 33's would probably be the smallest I would go on the Bronco.. so 4.70's it is.
 

John B+9

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Moving up tire sizes will decrease the RPM the engine turns for any given speed.(and visa versa)

Moving to Steeper gears will Increase the RPM the engine runs at any given speed (and Visa Versa)

So going to the steepest gear set, and then Decreasing tire sizes means your effectively increasing your gearing. I don’t have a calculator handy but your effective gear ratio is close to 4.9X at that point.
Nice! Great and easy to understand explanation. I took a cut at an Excel spreadsheet gear ratio / speed calculator for the 2.3L with 7MT. Posted on another thread if any interested in playing with it. (will continue with 2.3L and 2.7L with auto when I have a chance). Bottom line, shows that 33" with 4.46:1 rear end, and 'squatch with 35" and 4.70:1 rear both turn ~2,100 RPM at 70 MPH.
 

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Nice! Great and easy to understand explanation. I took a cut at an Excel spreadsheet gear ratio / speed calculator for the 2.3L with 7MT. Posted on another thread if any interested in playing with it. (will continue with 2.3L and 2.7L with auto when I have a chance). Bottom line, shows that 33" with 4.46:1 rear end, and 'squatch with 35" and 4.70:1 rear both turn ~2,100 RPM at 70 MPH.
Except that you can't have the 7MT with squatch and 35's....
 

TommyTwoTone

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Except that you can't have the 7MT with squatch and 35's....
Since when? Sasquatch comes with 35's, always, and can be put on any Bronco regardless of transmission. Only thing you can't get is 7MT with the 2.7L V6.

Correction: a few kind members pointed out that Sasquatch and MT is not an available combination at this time. I've never really been interested in a MT so I missed the announcement.
 
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