Should I regear

Garry

Garry

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New here, I recently installed a 2.5" X series rough country lift on my 2006 unlimited and put 33 inch tires on. It had not had a previous lift. I haven't changed the gear's and was wondering if I should, can someone give me some advice. It's an automatic. Thanks
 
It depends on what you currently have. If I had:

4.56 - no
4.10 - probably not
3.73 - probably yes
3.07 - hell yes

If you do end up replacing gears, 4.88 or 4.56 would work.
 
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For your particular 4-speed automatic transmission, you want nothing less than 4.88 gearing for 33" tires. Personally I would go with 5.13. No, not even 5.13 will cause excessive highway rpms. 5.13 would cause the rpms to be right in the middle of the optimal power band at 65-70 mph. 4.88 would just barely be in the optimal power band.

The engine will be lugging with higher ratios like 4.56 or 4.10 with your transmission and 33" tires.
 
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I looked at the grim jeepers website for gearing. Since you have the 42rle auto transmission, the overdrive gear is a really high ratio (0.69/1). Using that info, here are the rpms at 70 mph and running 33s:

4.10 - 2130 rpm
4.56 - 2369 rpm
4.88 - 2536 rpm
5.13 - 2666 rpm

As you can see, you can go with lower gears and still be fine at highway speeds of 2536 or 2666 rpm.

I have a 5-speed in my jeep and my overdrive gear is 0.79/1. I didn't know the auto had that much higher of an overdrive.
 
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Jerry Bransford said:
For your particular 4-speed automatic transmission, you want nothing less than 4.88 gearing for 33" tires. Personally I would go with 5.13. No, not even 5.13 will cause excessive highway rpms. 5.13 would cause the rpms to be right in the middle of the optimal power band at 65-70 mph. 4.88 would just barely be in the optimal power band.

The engine will be lugging with higher ratios like 4.56 or 4.10 with your transmission and 33" tires.
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How would you adjust that same set up with 31" tires?

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Jerry Bransford said:
Are 31's for sure as large of a tire as you'll ever run? If they are, I would not pay the big $$$ to regear unless your axles have the abysmal 3.07 ratio gears in them. :)
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It has 3.73. Trouble going over 50mph on freeway inclines. Great around town .

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Just be forewarned that there is a good chance with the 4:88 gear, 42rle and 33'in tire combo you stand a good chance of having a rotational vibration around 60 mph. There are a lot of people that have had that combo and have invested a lot of money into trying to get rid of the vibration.

Gearing wise it's a perfect match for the trans,tire combo. If part of your intentions with the jeep is to be able to drive it on expressway often it could be an issue.
 
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jeepndogs said:
Just be forewarned that there is a good chance with the 4:88 gear, 42rle and 33'in tire combo you stand a good chance of having a rotational vibration around 60 mph. There are a lot of people that have had that combo and have invested a lot of money into trying to get rid of the vibration.

Gearing wise it's a perfect match for the trans,tire combo. If part of your intentions with the jeep is to be able to drive it on expressway often it could be an issue.
Click to expand...

Interesting, I wonder why that is?
 
jeepndogs said:
Just be forewarned that there is a good chance with the 4:88 gear, 42rle and 33'in tire combo you stand a good chance of having a rotational vibration around 60 mph. There are a lot of people that have had that combo and have invested a lot of money into trying to get rid of the vibration.

Gearing wise it's a perfect match for the trans,tire combo. If part of your intentions with the jeep is to be able to drive it on expressway often it could be an issue.
Click to expand...

I haven't had any vibration, and I have the 42rle, running 33's, and have 5.13 gearing.
 
The 5speed manual can also fall victim to the over 60 mph vibes. I'm still chasing mine after re-gearing to 4.56's.
 
FTI76 said:
Disabling OD will make a big difference on the freeway.
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OK so I feel like a goof. Should have used the OD button my trip as it was always on. So testing today.

60mph = 1900rpm in OD
60mph = 2750rpm with OF off

60mph has not been adjusted for 31" tires.

Cruising at 2800+ rpm for 800 miles seems like it would be hard on the engine. ???

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I also have a 2006 LJ auto. When I purchased, it had 31s. I quickly went to 33s, and IMO, the stock gearing was acceptable. But I'm sure it would more optimal geared better. I didn't notice too much change from the 31 to 33. (I went to lighter tires tho)

If u ever think u will go to 35s, then regearing is needed. I'm running 35s right now, and it is a DOG. I turn OD off all the time. I hate it, but I have too many other things I want to spend money on first, so the regearing is taking a back seat.

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LeePTI said:
OK so I feel like a goof. Should have used the OD button my trip as it was always on. So testing today.

60mph = 1900rpm in OD
60mph = 2750rpm with OF off

60mph has not been adjusted for 31" tires.

Cruising at 2800+ rpm for 800 miles seems like it would be hard on the engine. ???

Sent from my SM-G928V using Tapatalk
Click to expand...
Turning off the od helps the highway situation, but it doesn't change the fact that all the lower gears are taller , and the puts more stress on the entire driveline. As for 2800 rpms, you could cross the country at that rpm, and the motor wouldn't even break a sweat.
 
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Andy5150 said:
Turning off the od helps the highway situation, but it doesn't change the fact that all the lower gears are taller , and the puts more stress on the entire driveline. As for 2800 rpms, you could cross the country at that rpm, and the motor wouldn't even break a sweat.
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x2

I was just talking with Jerry B about it yesterday. Our engines actually like being around the 2,800-3,000rpm range. By using a gear that's numerically too high for the situation, you actually make the engine work a lot harder than if you keep it in it's power band.
 
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LeePTI said:
It has 3.73. Trouble going over 50mph on freeway inclines. Great around town .

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Click to expand...

If 33's are good with 5.13, that leaves you with 4.88 for 31's.

4.56 would work just like 4.88 would work for 33's, but 4.88 wouldn't be insane with 31's and would actually be enjoyable so that is the route I would go.

Either way would be a nice jump from 3.73. If you're going to larger tires eventually, figure out what you want and then go for the best gears for that setup.
 
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I currently have 3.07 w/ a 5spd, 4.0, and 265/75s .... this thing dogs. Unfortunately, due to the previous owners lifting choices, I have to focus on the rear shaft and sye before moving into re-gearing.
 
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