Help needed on going to smaller tires

[Terrymo]

Sounds like you already had your mind made up. Try it and report back on how it went.

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[c20040215]

Sounds like you already had your mind made up. Try it and report back on how it went.

Like a woman holding two dresses, one on each hand and come to you, and asks you which one you like... Its a trap...

 

[Ratbert]

Like a woman holding two dresses, one on each hand and come to you, and asks you which one you like... Its a trap...

In this case she's asking which will make her look thinner while you know that both will do the opposite.

 

[Badunit]

If only there was a way for the transmission to raise the rpms (and thereby power output) at any time when needed...automatically. The idea that you need to gear to the moon to tow a 3k pound trailer is wild.

Your response oozes of condescending sarcasm. The difference between what you think is proper gearing and what I think is proper gearing is a matter of degree. I don't think 4.56 is "geared to the moon", I think it is the correct gearing for the engine, transmission, and tire size.

 

[Zandcwhite]

Your response oozes of condescending sarcasm. The difference between what you think is proper gearing and what I think is proper gearing is a matter of degree. I don't think 4.56 is "geared to the moon", I think it is the correct gearing for the engine, transmission, and tire size.

It's geared to the moon for 31s, no JL comes with a tire that small and all the ones on 32s come with 3.45s. The 8 speed has a 4.7-1 1st gear. You don't need to roll around in the 2nd overdrive gear at 2500 rpms to "stay in the power band". That's why none of them come that way stock. The JLU is rated to tow the same 3500lbs with 3.45s and 32s as it is with 4.56s and 35s.

 

[Badunit]

You don't need to roll around in the 2nd overdrive gear at 2500 rpms to "stay in the power band".

There is no "rolling around" at 60+ mph in a Jeep. It has a lot of drag and needs power/torque to push itself down the road.

2000 rpm is sufficient (barely). 2500 rpm is overkill except at high speeds.

I have no concerns about overdrive or "2nd overdrive". Transmission problems seem to be rare.

31" tires and 4.56 = 2000 rpm at 60 mph. 2300 at 70. 2500 at 75. I wouldn't want that running empty but we're talking about towing, with a Jeep. Probably won't be going all that fast.

 

[JL 2D]

If you’re going to do it, might as well go for it. 80mph in 1st gear should be fun.

 

[Wbino]

This is a thread I thought I'd never see on this forums.
Most here follow these rules:

 

[grimmjeeper]

You don't need to roll around in the 2nd overdrive gear at 2500 rpms to "stay in the power band".

What specifically is wrong with being in 2nd overdrive at 2,500 and why is it bad?

Please provide details.

 

[Zandcwhite]

What specifically is wrong with being in 2nd overdrive at 2,500 and why is it bad?

Please provide details.

The entire purpose of the 2nd overdrive is low rpms at low loads. I don't care if you're towing a trailer up a mountain, that's what downshifts are for. Odds are you're rolling down that mountain...in 8th gear...at <2k rpm as designed.

 

[grimmjeeper]

The entire purpose of the 2nd overdrive is low rpms at low loads. I don't care if you're towing a trailer up a mountain, that's what downshifts are for. Odds are you're rolling down that mountain...in 8th gear...at <2k rpm as designed.

So, you don't actually have any technical reason. Got it.

BTW, "as designed" was tuned specifically to get the maximum fuel mileage on an unrealistic test loop. There's no good engineering reason to run the 3.6 under 2,000 RPM on the freeway.

 

[Ratbert]

This is a thread I thought I'd never see on this forums.

Someone wanting to put shorter sidewalls on their Wrangler in hopes of increasing comfort? Yep, stupid shit happens here all the time.

 

[GabeBoyTheGreat]

So, you don't actually have any technical reason. Got it.

BTW, "as designed" was tuned specifically to get the maximum fuel mileage on an unrealistic test loop. There's no good engineering reason to run the 3.6 under 2,000 RPM on the freeway.

On the JK 5-speed, I would agree, but with the 8-speed, there is certainly an argument to be made to keep the RPMs below 2,000 to maximize fuel efficiency, which OP stated is one of the reasons he is considering doing this in the first place.

The argument being made, which I agree with in this case, is that if 8th gear is running at 2,500 RPM on the freeway, then you are losing out on potential fuel economy.

If 8th gear is running at 1,800 RPM, you will benefit from improved fuel economy under low load conditions (i.e. flat or slightly downhill). If the load increases, the tranny will downshift for more torque as needed. Therefore, there is no loss in towing performance.

I would even take it a step further and say that if 8th is running at 2,500 RPMs on the freeway that the tranny has gears that are wasted. The entire benefit of having 8 speeds is to have closer ratios in the shifts. Downshifting 1 gear in an 8-speed might only raise the RPMs by 200 whereas downshifting in a 4-speed might be closer to 800, which might be WAY more than you need. That is why people tend to put lower gears in the older Jeeps so their RPMs aren't jumping crazy high just to maintain speed on a slight incline.

Hope that makes sense.

 

[Zandcwhite]

So, you don't actually have any technical reason. Got it.

BTW, "as designed" was tuned specifically to get the maximum fuel mileage on an unrealistic test loop. There's no good engineering reason to run the 3.6 under 2,000 RPM on the freeway.

You literally just started the good reason to run low rpms at low loads... fuel economy. The better question is what do you gain by running 2500rpm all the time regardless of load? Gearing that low will have you in 8th gear by 40mph. What's the point of having an 8 speed if it's in top gear by half the speed limit? Might as well be an old school 3 speed with no overdrive.

 

[GabeBoyTheGreat]

You literally just started the good reason to run low rpms at low loads... fuel economy. The better question is what do you gain by running 2500rpm all the time regardless of load? Gearing that low will have you in 8th gear by 40mph. What's the point of having an 8 speed if it's in top gear by half the speed limit? Might as well be an old school 3 speed with no overdrive.

There is an F-150 at work with a 10-speed that I drive from time to time. It hits 10th gear by about 45 MPH. Such a waste of gears IMO.

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