Understanding Tire Size and Gearing

AndyA

AndyA

2006 TJ X
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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:

https://tirecalculator.com/225-75r15-tires
This chart shows for 28" tires and 3.73 gearing the RPMs at 65MPH are 2909:

https://myjeeptj.com/wrangler-write-ups/gear-ratio-to-tire-size-chart/
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]https://tirecalculator.com/225-75r15-tires[/URL]

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

[URL]https://myjeeptj.com/wrangler-write-ups/gear-ratio-to-tire-size-chart/[/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]https://tirecalculator.com/225-75r15-tires[/URL]

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

[URL]https://myjeeptj.com/wrangler-write-ups/gear-ratio-to-tire-size-chart/[/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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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][URL][URL]https://tirecalculator.com/225-75r15-tires[/URL][/URL][/URL]

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

[URL][URL][URL]https://myjeeptj.com/wrangler-write-ups/gear-ratio-to-tire-size-chart/[/URL][/URL][/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
Click to expand...

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][URL]https://tirecalculator.com/225-75r15-tires[/URL][/URL]

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

[URL][URL]https://myjeeptj.com/wrangler-write-ups/gear-ratio-to-tire-size-chart/[/URL][/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...

I've got a Jeep Wrangler TJ 4.0. 3 speed auto We put 33s on it and tried it on the road. She won't achieve third gear. What tire size could you go up to that would allow the gearbox to run correctly? I know it really wants a re-gear to a 4:10. But .$£$£. This vehicle is used for overlanding not used for rock climbing.
 
maz said:
I've got a Jeep Wrangler TJ 4.0. 3 speed auto We put 33s on it and tried it on the road. She won't achieve third gear. What tire size could you go up to that would allow the gearbox to run correctly? I know it really wants a re-gear to a 4:10. But .$£$£. This vehicle is used for overlanding not used for rock climbing.
Click to expand...
You mean it absolutely will not shift into 3rd gear no matter what? Has anyone messed around with the TV (throttle valve) cable adjustment? The TV cable is what adjusts the shift points and if it's adjusted too tightly (it should be just taut at idle rpms) it can prevent upshifting at appropriate rpms.

What is your axle ratio? If you don't know it, the ratio is embossed into a steel tag bolted onto the differential covers at their 9 o'clock position.

This tag is indicating the axle has 3.73 gearing.

Axle Ratio.jpg
 
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Jerry Bransford said:
You mean it absolutely will not shift into 3rd gear no matter what? Has anyone messed around with the TV (throttle valve) cable adjustment? The TV cable is what adjusts the shift points and if it's adjusted too tightly (it should be just taut at idle rpms) it can prevent upshifting at appropriate rpms.

What is your axle ratio? If you don't know it, the ratio is embossed into a steel tag bolted onto the differential covers at their 9 o'clock position.

This tag is indicating the axle has 3.73 gearing.

View attachment 484756
Click to expand...

Thanks for the reply no
Jerry Bransford said:
You mean it absolutely will not shift into 3rd gear no matter what? Has anyone messed around with the TV (throttle valve) cable adjustment? The TV cable is what adjusts the shift points and if it's adjusted too tightly (it should be just taut at idle rpms) it can prevent upshifting at appropriate rpms.

What is your axle ratio? If you don't know it, the ratio is embossed into a steel tag bolted onto the differential covers at their 9 o'clock position.

This tag is indicating the axle has 3.73 gearing.

View attachment 484756
Click to expand...

Thanks for getting back 3.73. I believe . There is no tag on the axle so I'll take the case off and Count the teeth and work it out, but nobody's touched anything. We literally bought the vehicle. Drove it about 90 miles home. All was good but somebody had put 20 inch wheels on it. Horrendous. 😳 We put a 4" lift on, then fitted 33.12 .17s on 33 inch tires. Plus fitted SYE kit.. Took her out on the road ,she selects the first gear selects the second gear , but the rpm can go up to 4000 RPM won't go into third. Yet it was absolutely fine before we change the wheels and tires
 
The other axle might have the tag. Have you checked the transmission ATF or yet? The engine MUST be running and the transmission MUST be in Neutral while checking its ATF level.
 
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Jerry Bransford said:
The other axle might have the tag. Have you checked the transmission ATF or yet? The engine MUST be running and the transmission MUST be in Neutral while checking its ATF level.
Click to expand...

Yes I did an oil change on the gearbox first thing when I got it ,so I know that's spot on. Everyone is coming back with, re- gear it to 4:10. The thing is it spins the wheels off the mark and goes up into second and pulls strong. Just won't go into third. But I'll double check the oil level.
 
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