Do I need to tweak tire size? (help)

[Sw00per]

Hi All,

So just got back from a mini road trip and noticed an issue with staying in 8th over 75 mph (gearing\torque issue?) and I also noticed it at slightly lower speeds (70ish) a few times.

I upgraded to 35" BFG K0's and have my size set to 33.4.

On the freeway on a flat road as soon as I went over 75mpg it downshifts to 7 and stays there i.e. 76mph. One difference is a bit of extra weight i.e. two passengers.

I am wondering if I tweak the tire size a bit to optimize if that will help if so I am not sure if I need to go up or down..?

Thanks in advance!

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[Escape.idiocracy]

I match the tire size with two alternative speed reading devices- one being a Uniden R4 and the other being an iPhone- speed app or Waze.

As long as the speedometer is correct, the shifting/gear is being done by the computers as needed to “manage” the torque required to maintain speed.

If 7th gear doesn’t meet your needs, re-gearing can put things back to “stock” though, for 35” tires… might be a good time to upgrade. Wheel and tire weights are an important factor…. My in-law is running black rhino wheels and some Heavy MT’s that weigh in at 115#’s….. our 37’s weigh in at 105#’s ??‍ all starts to add up and make a difference.

 

[Sw00per]

Thanks. I did factor in weight of the wheel and tire combo so they are very close.

I am interested in understanding the relationship between the tire size, rotational speed and torque to understand how the computer determines to gear.

I have it stuck in my head that tire size is impacting getting in to 8th when I think it should be.

 

[WILDHOBO]

Thanks. I did factor in weight of the wheel and tire combo so they are very close.

I am interested in understanding the relationship between the tire size, rotational speed and torque to understand how the computer determines to gear.

I have it stuck in my head that tire size is impacting getting in to 8th when I think it should be.

What are the details of your truck? Sport, sport s, max tow, overland, Mohave, rubicon, diesel or not? That’ll give you transfer case ratio and gear ratio from stock. It really depends on what yours currently is to answer the questions. Tire size programming matters for Speedo calibration for sure. Gearing is big for getting your gears back.

 

[Gren71]

It’s likely the tire size / weight / tread pattern that is getting you in 7th. Also the speed. Over 65mph the Jt takes a huge aerodynamic hit and it really drags bad. especially if you’re running 3.73 gears with 35’s.

 

[Lunentucker]

I run the same tires on stock rims and a 2 inch lift.
My rolling PSI is 32. I set the tire size using ECRI and the dash speed matches Waze perfectly.
7th at that speed is very common. If I'm on a long flat stretch of Interstate I'll sometimes manually shift into 8th, but the MPG difference is negligible.
One thing @ShadowsPapa taught me on here is that these engines are pretty good about knowing what they need to run optimally, and slightly higher RPMs are not necessarily bad for them.

Turn on your Off Road Pages and watch your oil PSI, keeping in mind that these things oil from the top down, and there's a lot of need for fresh cooled oil on the top end of the block.

If you leave the shifter in manual override, it will still shift when it must to maintain speed.
That's a good way of seeing what the parameters are in the computer.
You can override with cruise control on as well.

 

[ShadowsPapa]

Hi All,

So just got back from a mini road trip and noticed an issue with staying in 8th over 75 mph (gearing\torque issue?) and I also noticed it at slightly lower speeds (70ish) a few times.

I upgraded to 35" BFG K0's and have my size set to 33.4.

On the freeway on a flat road as soon as I went over 75mpg it downshifts to 7 and stays there i.e. 76mph. One difference is a bit of extra weight i.e. two passengers.

I am wondering if I tweak the tire size a bit to optimize if that will help if so I am not sure if I need to go up or down..?

Thanks in advance!

Be careful about not setting tire size accurately if anything should come up for warranty. They may use it as an excuse to not cover something.

Not sure why you are trying to trick the systems, but these are configured for best results - unless you are a better engineer than Jeep hires.
You may get into trouble with heat rejection among other things.
I'd frankly set the tire size correctly and move on.
(seems to me some people prefer to regear once they get to 35 or 37)

I have several miles of highway where my Overland with stock tire sizes doesn't see 8th gear at 65.
I've driven all the way from Waukee to Altoona in 6th and 7th gear because of some wind.
Mine will routinely run at 2400-2500 RPM on the highway if there's a wind and incline.
Any wind or hills is going to see these stay out of 8th - and that's fine. Don't try to trick it.

7th at that speed is very common. If I'm on a long flat stretch of Interstate I'll sometimes manually shift into 8th, but the MPG difference is negligible.

I believe it. Sounds familiar.

I have it stuck in my head that tire size is impacting getting in to 8th when I think it should be.

My gut tells me you are over-thinking it and believing it should be in 8th when it wouldn't be with most of our Jeeps. There's just not the torque to do that if encountering any wind resistance at all or inclines.

I'd honestly set the tire size where it belongs.
The transmission should go into learn mode, may shift weird, even harsh for a while, then settle back down and leave it alone from there. it's doing it right - you'll not do any good trying to "trick" it.
8th is a 3rd overdrive gear and when these get down that far in RPM, they aren't at their best.

 

[Sw00per]

What are the details of your truck? Sport, sport s, max tow, overland, Mohave, rubicon, diesel or not? That’ll give you transfer case ratio and gear ratio from stock. It really depends on what yours currently is to answer the questions. Tire size programming matters for Speedo calibration for sure. Gearing is big for getting your gears back.

Mojave.

 

[Sw00per]

I run the same tires on stock rims and a 2 inch lift.
My rolling PSI is 32. I set the tire size using ECRI and the dash speed matches Waze perfectly.
7th at that speed is very common. If I'm on a long flat stretch of Interstate I'll sometimes manually shift into 8th, but the MPG difference is negligible.
One thing @ShadowsPapa taught me on here is that these engines are pretty good about knowing what they need to run optimally, and slightly higher RPMs are not necessarily bad for them.

Turn on your Off Road Pages and watch your oil PSI, keeping in mind that these things oil from the top down, and there's a lot of need for fresh cooled oil on the top end of the block.

If you leave the shifter in manual override, it will still shift when it must to maintain speed.
That's a good way of seeing what the parameters are in the computer.
You can override with cruise control on as well.

The RPMs in 8th are at 2K when it goes into 7th 2500ish rpms. Maybe the altitude and extra two passengers. I set the tires same as you and used GPS. The speedo is accurate. Maybe I need to reset my expectations..?

 

[Sw00per]

Be careful about not setting tire size accurately if anything should come up for warranty. They may use it as an excuse to not cover something.

Not sure why you are trying to trick the systems, but these are configured for best results - unless you are a better engineer than Jeep hires.
You may get into trouble with heat rejection among other things.
I'd frankly set the tire size correctly and move on.
(seems to me some people prefer to regear once they get to 35 or 37)

I have several miles of highway where my Overland with stock tire sizes doesn't see 8th gear at 65.
I've driven all the way from Waukee to Altoona in 6th and 7th gear because of some wind.
Mine will routinely run at 2400-2500 RPM on the highway if there's a wind and incline.
Any wind or hills is going to see these stay out of 8th - and that's fine. Don't try to trick it.


I believe it. Sounds familiar.


My gut tells me you are over-thinking it and believing it should be in 8th when it wouldn't be with most of our Jeeps. There's just not the torque to do that if encountering any wind resistance at all or inclines.

I'd honestly set the tire size where it belongs.
The transmission should go into learn mode, may shift weird, even harsh for a while, then settle back down and leave it alone from there. it's doing it right - you'll not do any good trying to "trick" it.
8th is a 3rd overdrive gear and when these get down that far in RPM, they aren't at their best.

Yup I am thinking I am guilty of over thinking and creating a problem where there is none... at work this is job security here not so much... ? ?

 

[WILDHOBO]

The RPMs in 8th are at 2K when it goes into 7th 2500ish rpms. Maybe the altitude and extra two passengers. I set the tires same as you and used GPS. The speedo is accurate. Maybe I need to reset my expectations..?

Those RPMs and gears sound perfect. I have a friend with a Mohave running 35 x 12.5’s with no issues. I think you’re perfect. I wouldn’t regear unless you decide to go up to 37s or bigger.

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