How to choose your axle gear ratio based on tire size

Used2jeep

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Phosvitin

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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

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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

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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

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Choosing gear ratios is easy. Choose the lowest (highest numerically) that the manufacturer offers for your tire/wheel package.
 

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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

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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

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grimmjeeper

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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.
 
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JT58Bronc

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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.
 

Beachin 74

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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

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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.
 

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