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Understanding Tire Size and Gearing

andya

andya

TJ Enthusiast
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Joined
Apr 30, 2023
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Location
Tucson, AZ
I am learning about the relationship between the size of the tires and gearing to understand how it fits together, Do I have this right?

My TJ came from the factory with 3.73 gears and P225/75R15 tires

Those original tires have a diameter of 28.3" according to this:

tirecalculator.com

225/75R15 Tires

225/75R15 tire specs, conversion, pressure chart, replacement sizes.
tirecalculator.com tirecalculator.com

This chart shows for 28" tires and 3.73 gearing the RPMs at 65MPH are 2909:

myjeeptj.com

Gear Ratio To Tire Size Chart - My Jeep TJ

Upgrading your axles? Find your ideal gearing in this TJ gear chart.
myjeeptj.com

To achieve the closest RPMs (and therefore similar performance and fuel economy) for various tire sizes the gearing needs to be:

30" = 3.91 (2847 RPM)
31" = 4.11 (2896)
32" = 4.27 (2915)
33" = 4.27 (2825)

?

Thanks, Andy
 
Most people here use Thee Grim Jeeper, here's the link

http://www.grimmjeeper.com/gears.html

Works if you actually measure the rolling diameter of your wheel/tire.
Measure from center of tire to ground and multiply it by 2. This gives the actual rolling diameter.
Manufacturers can differ in rolling diameter.
This method is the most accurate.
 
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andya said:
I am learning about the relationship between the size of the tires and gearing to understand how it fits together, Do I have this right?

My TJ came from the factory with 3.73 gears and P225/75R15 tires

Those original tires have a diameter of 28.3" according to this:

[/URL]

This chart shows for 28" tires and 3.73 gearing the RPMs at 65MPH are 2909:

[/URL]

To achieve the closest RPMs (and therefore similar performance and fuel economy)
Click to expand...
you also have to consider the increased weight and rolling resistance of larger/wider tires
 
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The great thing about the grimmjeeper is that it allows you to change all the variables in the drivetrain. (except engine:oops:)
 
N757KF said:
The great thing about the grimmjeeper is that it allows you to change all the variables in the drivetrain. (except engine:oops:)
Click to expand...

And allows you to make false assumptions about which ratios will work well.since none of it is in context
 
expert1.jpg
 
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andya said:
I am learning about the relationship between the size of the tires and gearing to understand how it fits together, Do I have this right?

My TJ came from the factory with 3.73 gears and P225/75R15 tires

Those original tires have a diameter of 28.3" according to this:

[/URL]

This chart shows for 28" tires and 3.73 gearing the RPMs at 65MPH are 2909:

[/URL]

To achieve the closest RPMs (and therefore similar performance and fuel economy) for various tire sizes the gearing needs to be:

30" = 3.91 (2847 RPM)
31" = 4.11 (2896)
32" = 4.27 (2915)
33" = 4.27 (2825)

?

Thanks, Andy
Click to expand...

That article has some flaws. For example, gearing lower will not cause you to have worse mpg. That’s a result of the bigger tires and lift.

There are more factors to consider when choosing a gear ratio than just tire size. Transmission, actual rolling diameter, and rotational mass. Most of these point to a deeper than expected gear ratio.

Check the Grimm Jeeper Gear page out. Type in your specs and do some comparisons. Check back here for real world experience and you’ll get some good feedback from people who know.
 
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