GrimmJeeper Gear Ratio Calculator

[MudderNuker]

I'm I doing this right? I'm trying to decide which tires to pick for my '21 JTW with 3.73 gears. Stock 255/75r27 are 32.1 inches. I'm looking at moving up to a 295/70r17 which is around 33.3 inches. I selected the Dodge 850RE tranmission and the Select Trac MP3022. Are this the correct choices for the 8 speed?

When I enter the new tire size 295s the engine would be revving only 63 RPM less than running the 255s with the stock 32s. To me that seems negligible so I'm wondering if I'm doing something wrong with my inputs.

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[Wheelin98TJ]

The Dodge 850RE has different ratios. Your 850RE is a 0.67 overdrive.

But your RPM difference is accurate.

You can calculate another way:

255/75/17 (32.06" tall) to 295/70/17 (33.26" tall) is a 1.2" change or 3.74%.​

​

1,688 RPM x 1.0374 = 1,751 RPM​

​

1,751 RPM - 1,688 RPM = 63 RPM difference​

 

[MudderNuker]

The Dodge 850RE has different ratios. Your 850RE is a 0.67 overdrive.

But your RPM difference is accurate.

You can calculate another way:

255/75/17 (32.06" tall) to 295/70/17 (33.26" tall) is a 1.2" change or 3.74%.​

​

1,688 RPM x 1.0374 = 1,751 RPM​

​

1,751 RPM - 1,688 RPM = 63 RPM difference​

Thanks for the reply. So going by that, will the 63 RPM difference make the JT feel sluggish or prevent it from using 8th gear. Right now as is, it feel plenty peppy.

 

[Wheelin98TJ]

Thanks for the reply. So going by that, will the 63 RPM difference make the JT feel sluggish or prevent it from using 8th gear. Right now as is, it feel plenty peppy.

I don't think you'll have any issue with the acceleration.

And I'd think you'd still get into 8th plenty often, but I'm just guessing on that. I don't have any experience with that tire size. Hopefully someone else with experience can give some info.

 

[MudderNuker]

Honestly, while I love the looks of a 37" tire, I don't want to regear (been there, done that) or lift any higher than 2". I don't commute since I retired a few months back so I'm not worried about gas mileage but don't want to spend $3k on wheels and tires plus a lift and labor. That is why I'm trying to stay at a reasonable tire size. Sort of a happy medium with what I currently have, 3.73 and 1.5" TF leveling.

 

[AverageJoe]

I installed 285/75r17's (33.9") on my Willys. Acceleration is fine. My truck shifts into 8th on level ground, but any incline will make it downshift. Hope this helps with your decision.

 

[MudderNuker]

I installed 285/75r17's (33.9") on my Willys. Acceleration is fine. My truck shifts into 8th on level ground, but any incline will make it downshift. Hope this helps with your decision.

I think my my choices are between 295/70r17 or 285/75r17. I never see 7 or 8 for most of my driving around our mountain property. Heck I'm not even sure the truck ever shifts into 6th. We are at 7,300' in elevation and the speed limits around here are 55 tops. It's all hills around here and some are very steep. Considering the elevation, my JT with 3.73 still does fine with the 32 MTS, but I might be pushing my luck with 285/75s. I'm just waiting for a killer Black Friday deal today to pull the trigger on a set of tires.

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