Sponsored

How to choose your axle gear ratio based on tire size

Used2jeep

Banned
Black Diamond
Banned
Banned
First Name
Dave
Joined
Jul 16, 2020
Threads
46
Messages
6,191
Reaction score
12,813
Location
Massachuvian
Vehicle(s)
2007 Crown Vic P71
Your Bronco Model
Black Diamond
Clubs
 

Phosvitin

Big Bend
Active Member
Joined
Sep 13, 2020
Threads
6
Messages
44
Reaction score
149
Location
Madison WI
Vehicle(s)
Toyota matrix and honda civic
Your Bronco Model
Big Bend
This math equation in the video is horribly arbitrary. By his reasoning he only needs a 3.49 ratio to go to 33" tires. If I do the calculation for my base bronco with a 4.46 gear ratio from 30" to 32" I would apparently need to upgrade my gear ratio to at least 4.7 -.-
 

Erock

Badlands
Well-Known Member
First Name
Eric
Joined
Jan 30, 2021
Threads
1
Messages
2,171
Reaction score
5,104
Location
US
Vehicle(s)
2018 F150 Alpine edition
Your Bronco Model
Badlands
Clubs
 
It’s a decent equation since he kept it in perspective of the individual vehicles ( ex: same weight and manufacturers intended uses). There are a lot more factors To consider than just tire size.
 

Drex

Banned
Badlands
Banned
Banned
First Name
Jake
Joined
Jan 4, 2020
Threads
11
Messages
2,338
Reaction score
7,076
Location
various, construction engineer.
Vehicle(s)
'13 SLK55 AMG, '15 Indian Chief, '15 WRX
Your Bronco Model
Badlands
Didn't watch the video, but the circumference difference between a 30" and a 32" is what, about 14%? So getting some of that back with a 5% deeper gear seems conservative if anything.
 

Beachin 74

Outer Banks
Well-Known Member
Joined
Jun 22, 2020
Threads
9
Messages
974
Reaction score
2,615
Location
Delaware
Vehicle(s)
1974 Bronco, 2015 F-150, 2022 OBX
Your Bronco Model
Outer Banks
Choosing gear ratios is easy. Choose the lowest (highest numerically) that the manufacturer offers for your tire/wheel package.
 

Sponsored

MaddMax

Badlands
Member
Joined
Jan 16, 2021
Threads
1
Messages
18
Reaction score
72
Location
Dalton, GA
Vehicle(s)
2017 Audi S5, 1970 Chevy Impala
Your Bronco Model
Badlands
Considering the Badlands Mini/Skinny Squatch:

1) Non-Sas Badlands 2.7 Auto
2) 305/70/17 or 285/75/17 tires - Both 33.8" vs. stock 32.8"
3) Small level

Using the equation in the video i get 4.59 as the "optimal gear ratio" vs 4.46 for Badlands auto. Seems like no re-gearing would necessary to run 34s and the difference shouldn't be very noticeable.
 

Gamecock

Banned
Badlands
Banned
Banned
Joined
Jul 16, 2020
Threads
14
Messages
3,009
Reaction score
10,259
Location
Virginia
Vehicle(s)
2021 Bronco Badlands / Sasquatch
Your Bronco Model
Badlands
This math equation in the video is horribly arbitrary. By his reasoning he only needs a 3.49 ratio to go to 33" tires. If I do the calculation for my base bronco with a 4.46 gear ratio from 30" to 32" I would apparently need to upgrade my gear ratio to at least 4.7 -.-
Not sure what is arbitrary about it. You bringing up what he says he needs for 33s on his Jeep is of no consequence to what you need on a Bronco....because the transmissions and transfer case are completely different, and are just as important in the formulas as the final ratio. His formula is sound for adding tire size to a given vehicle to keep RPMs in stock range.
 

North7

Well-Known Member
Joined
Feb 21, 2020
Threads
111
Messages
7,265
Reaction score
25,477
Location
North Texas
Vehicle(s)
SUV
Your Bronco Model
Undecided
Clubs
 

grimmjeeper

Well-Known Member
First Name
Roy
Joined
Nov 9, 2020
Threads
1
Messages
80
Reaction score
134
Location
Castle Rock, CO
Website
www.grimmjeeper.com
Vehicle(s)
Jeeps and a Subaru
Your Bronco Model
Undecided
That chart leaves a lot to be desired. From their page:

This chart is based on 65 MPH and a gear ratio of 1:1, on a manual transmission in 4th gear
Problem #1: Most freeway speeds are 75 MPH.

Problem #2: "In 4th gear" assumes a 4 speed manual (or 5 speed with overdrive). I've driven 3, 4, 5, and 6 speed manual transmissions. Some with overdrives. At least one with two overdrives. So "4th gear" isn't always the same thing in different transmissions. Beyond that, overdrive varies from one transmission to another. And the overdrive ratio matters.

Problem #3: Probably the biggest problem, is that all tires are smaller than their advertised size. A 35x12.50/17 tire has an actual rolling diameter usually under 34" tall. So when you're trying to pick gears for your "35's", you won't get the results you think you will if you plug 35 inches into a chart or calculator.

Problem #4: Different engines behave differently. Some engines operate best at low and mid range RPMs. Especially diesels. Then there are high revving engines that don't do well at lower RPMs. So the color coding may not line up with what your engine does well at. An older diesel may be happiest at 1,800 RPM which is off the color numbers on that chart. On the other hand, you Suzuki Samurai 4 banger really thrives up above 3,000 RPM.

So what it comes down to is that you can't make a "one size fits all" chart to suggest what a good ratio is for any given 4x4. You need more information.

My XJ on 32's (265/75R16) and 4.56 gears runs around 2,800 RPM at 75. That's nowhere near the numbers that chart would suggest. For my Jeep, the 4.56 gears is actually a great balance of highway driveability/fuel economy and off road/towing performance. But according to that chart, it's not.

So take any chart you see with a grain of salt. Using a calculator or a chart that is built using more information (like your overdrive ratio) and better data (actual tire diameter) will give you better information.
 
Last edited:

JT58Bronc

Base
Well-Known Member
First Name
JT
Joined
Jul 14, 2020
Threads
18
Messages
1,174
Reaction score
1,435
Location
Fl
Vehicle(s)
2022 Bronco, 2021 Miata track Edition
Your Bronco Model
Base
Looks good, thanks for sharing the information. There are a lot of lifted trucks in my area. I don't think very many go as far as to change gearing after installing lifts and huge tires. It makes a difference in both performance and gas mileage.
 

Sponsored

Aman

Badlands
Well-Known Member
First Name
Aman
Joined
Jul 6, 2020
Threads
5
Messages
1,469
Reaction score
4,048
Location
Northern NJ
Vehicle(s)
1998 M3, 2008 Armada, and 2022 Badlands ManSquatch
Your Bronco Model
Badlands
Clubs
 
Last edited:

Beachin 74

Outer Banks
Well-Known Member
Joined
Jun 22, 2020
Threads
9
Messages
974
Reaction score
2,615
Location
Delaware
Vehicle(s)
1974 Bronco, 2015 F-150, 2022 OBX
Your Bronco Model
Outer Banks
That chart leaves a lot to be desired. From their page:



Problem #1: Most freeway speeds are 75 MPH.

Problem #2: "In 4th gear" assumes a 4 speed manual (or 5 speed with overdrive). I've driven 3, 4, 5, and 6 speed manual transmissions. Some with overdrives. At least one with two overdrives. So "4th gear" isn't always the same thing in different transmissions. Beyond that, overdrive varies from one transmission to another. And the overdrive ratio matters.

Problem #3: Probably the biggest problem, is that all tires are smaller than their advertised size. A 35x12.50/17 tire has an actual rolling diameter usually under 34" tall. So when you're trying to pick gears for your "35's", you won't get the results you think you will if you plug 35 inches into a chart or calculator.

Problem #4: Different engines behave differently. Some engines operate best at low and mid range RPMs. Especially diesels. Then there are high revving engines that don't do well at lower RPMs. So the color coding may not line up with what your engine does well at. An older diesel may be happiest at 1,800 RPM which is off the color numbers on that chart. On the other hand, you Suzuki Samurai 4 banger really thrives up above 3,000 RPM.

So what it comes down to is that you can't make a "one size fits all" chart to suggest what a good ratio is for any given 4x4. You need more information.

My XJ on 32's (265/75R16) and 4.56 gears runs around 2,800 RPM at 75. That's nowhere near the numbers that chart would suggest. For my Jeep, the 4.56 gears is actually a great balance of highway driveability/fuel economy and off road/towing performance. But according to that chart, it's not.

So take any chart you see with a grain of salt. Using a calculator or a chart that is built using more information (like your overdrive ratio) and better data (actual tire diameter) will give you better information.
Most car folks know all this and the that chart is a 1 to 1 final drive ratio. I'm going to take a guess your XJ has overdrive therefore the rpm difference.
 

grimmjeeper

Well-Known Member
First Name
Roy
Joined
Nov 9, 2020
Threads
1
Messages
80
Reaction score
134
Location
Castle Rock, CO
Website
www.grimmjeeper.com
Vehicle(s)
Jeeps and a Subaru
Your Bronco Model
Undecided
Most car folks know all this and the that chart is a 1 to 1 final drive ratio.
You'd be surprised. A lot of people will miss that those charts are for a 1:1 ratio because they just look at the chart and don't read the text that goes with it. And a lot of people don't understand how advertised tire sizes don't match actual rolling diameter. Even fewer people think about why that's important until you explain it to them for the first time. Even veteran "car folks" don't really think about it all that much.

I'm going to take a guess your XJ has overdrive therefore the rpm difference.
Yep. But if you just apply overdrive you still get the wrong numbers. My 265/75R16 tires, which people typically think of as "32's", have an advertised diameter of 31.7". But in reality, the actual on the road diameter of the tire is closer to 30.7". The "35's" on my old Wrangler were actually about 33.5" in diameter. That makes a very big difference.
 

ryansbronco

Badlands
Well-Known Member
First Name
Ryan
Joined
Jan 20, 2021
Threads
3
Messages
52
Reaction score
65
Location
Edmonds wa
Vehicle(s)
2017 f3506.7L
Your Bronco Model
Badlands
Looks like 4.88s or 5.13s for me. Going with 37in tires.
 

peterh

Well-Known Member
Joined
Jul 15, 2020
Threads
64
Messages
1,193
Reaction score
1,946
Location
ny
Vehicle(s)
jeep wrangler jlu 2018
Your Bronco Model
Undecided
Clubs
 
Considering the Badlands Mini/Skinny Squatch:

1) Non-Sas Badlands 2.7 Auto
2) 305/70/17 or 285/75/17 tires - Both 33.8" vs. stock 32.8"
3) Small level

Using the equation in the video i get 4.59 as the "optimal gear ratio" vs 4.46 for Badlands auto. Seems like no re-gearing would necessary to run 34s and the difference shouldn't be very noticeable.
This is where I am at - looking to put 285/75/17 on my BL. I don't think going to 4.7 is necessary - thoughts from people with knowledge?
Sponsored

 
 


Top