Help me settle the regear ratio question

[omnitonic]

I have the 3.6L 6MT and 3.45 gears. This table neatly summarizes my options:

Running at 70 mph​

	4th	5th	6th
3.45 (current)	2531	2055	1822
4.10	2755	2232	1984
4.56	3064	2482	2206

Sitting on my current wheels and tires, I'm getting an overall average of 20 mpg I currently use 6th and 5th on the way to work, and 5th and 4th (and occasionally 3rd) on the way home. The grades are much steeper on the way home.

It looks like 4.10 is a good compromise in terms of fuel economy. I get my cruising RPMs up a lot closer to the sweet spot at 2000 while still having some room to lug along if I want. If I already had 4.10 gears, I would probably just stay there.

If I go to 4.56, I end up turning a little high in any of the gears I would normally use at 70 mph. I currently spend a lot of time in 5th, so I guess it's really only about 150 RPM. On the other hand, when I currently do downshift to 4th, I can see those fuel economy numbers drop like a stone, and that is only 300 RPM higher than I would be running in the new 6th gear.

One of the things that's making this kind of hard to puzzle out is that there is basically no flat stretch of road anywhere around here so I can cruise at 70 on flat ground in each of the three gears. I notice I get worse gas mileage at wide open throttle than I do if I'm at, say, 50% throttle. Driving around in something close to the equivalent of my current 4th gear all the time might not be as bad as I think. I am currently only seeing the fuel economy plummet whenever I'm in a wide open throttle situation. At other times, I am normally in 5th or even 6th.

So that's why I'm writing all this up. I'm looking for folks to share experiences they've had. I have a feeling y'all are going to talk me into the 4.56 gears. I'm leaning that way, but I need more convincing.

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[GATORB8]

This is on stock tires?

Remember you'll likely be in a different transmission gear for the same part of the drive with the regear. 4.10s in 6th will be close to 3.45s in 5th, and 4.56s in 4th will be like 3.45s in 6th. You'll be effectively losing 6th gear with either (meaning you won't have as "high" of an overdrive, not that it won't be usable).

Gear	Ratio	3.45​	3.73​	4.1​	4.56​
1​	5.13​	17.70​	19.13​	21.03​	23.39​
2​	2.63​	9.07​	9.81​	10.78​	11.99​
3​	1.53​	5.28​	5.71​	6.27​	6.98​
4​	1​	3.45​	3.73​	4.10​	4.56​
5​	0.81​	2.79​	3.02​	3.32​	3.69​
6​	0.72​	2.48​	2.69​	2.95​	3.28​

 

[Some Random Guy]

It sounds like you’re in a good spot if you actually see 6th gear. If you’re not significantly upgrading tires, I don’t think it is worth the cost.

 

[omnitonic]

This is on stock tires?

The stock figures are on stock tires. The 4.10 and 4.56 figures are on the 35s.

Remember you'll likely be in a different transmission gear for the same part of the drive with the regear.

Right. I'm basically deciding by how much to shove 6th off the chart. It's only very marginally useful here anyway, and once I get the 35s on, it will no longer be useful at all.

The 4.10 option is more conservative with respect to fuel economy, but will I be happy spending $2,000 to end up there? It seems like I read more posts from people on here who wish they had gone lower, instead of the other way around.

If I already had 4.10 gears, that's how I would go for sure. Just stay put and do nothing and live with it. Since I have the chance to choose the Perfect Ratio™, I'm kind of stuck on this. I'm a nerd with OCD, and I've been abusing the hell out of the GrimmJeeper's calculator. That's why I'm trying to get a reality check from people who have more first-hand experience with this stuff.

 

[Lohkay]

From my experience: 4.10 for 33s, 4.56 for 35s, 4.88 for 37s. That is on an auto tranny though.

 

[omnitonic]

It sounds like you’re in a good spot if you actually see 6th gear. If you’re not significantly upgrading tires, I don’t think it is worth the cost.

I only see 6th gear when I'm mostly going downhill, and it probably doesn't even matter whether I'm in 6th or 5th or whatever, because I'm at 0% throttle, and just using the gears to hold me back. I consider the switch from 32s to 35s a significant upgrade, although this is the first time in my life I ever changed a tire size, and maybe I should have held out for the 40s like Lite Brite. All I need is a decently hot girl and a Youtube channel, right?

 

[plex]

From my experience: 4.10 for 33s, 4.56 for 35s, 4.88 for 37s. That is on an auto tranny though.

And for manual, you have to kick up one notch.

Also this does not factor in unsprung weights, towing, etc.

 

[GATORB8]

The stock figures are on stock tires. The 4.10 and 4.56 figures are on the 35s.

Right. I'm basically deciding by how much to shove 6th off the chart. It's only very marginally useful here anyway, and once I get the 35s on, it will no longer be useful at all.

The 4.10 option is more conservative with respect to fuel economy, but will I be happy spending $2,000 to end up there? It seems like I read more posts from people on here who wish they had gone lower, instead of the other way around.

If I already had 4.10 gears, that's how I would go for sure. Just stay put and do nothing and live with it. Since I have the chance to choose the Perfect Ratio™, I'm kind of stuck on this. I'm a nerd with OCD, and I've been abusing the hell out of the GrimmJeeper's calculator. That's why I'm trying to get a reality check from people who have more first-hand experience with this stuff.

Looks like the 4.56s on 35s will end up pretty close to a stock Rubicon with 4.10s, maybe do a test drive of one (if you can find one). Obviously they'll be some more drag from the heavier tires, but may get you a good idea of how it should act with shifting.

 

[grimmjeeper]

I'm kind of stuck on this. I'm a nerd with OCD, and I've been abusing the hell out of the GrimmJeeper's calculator. That's why I'm trying to get a reality check from people who have more first-hand experience with this stuff.

I know all about being a nerd with OCD. It's why I wrote the calculator.

Thing is, it's hard to know just from the numbers on a computer screen. There are several other things that factor into the decision. The larger tires make the biggest difference but then there's the marginal effects of added weight and a larger contact patch from a wider tire.

Beyond that, you have to account for the fact that the engineers who set it up in the first place were balancing factors that may not apply to your desires/needs. CAFE standards, production cost optimization, etc.

I had a JK with the 6 speed. I went with 4.56 gears when I did my 35's. I think it would have been better with 4.88s, especially when I moved from <1,000 ft altitude to >6,000. The 6 speed in the JL has a slightly deeper overdrive than the JK does. But it has a deeper 1st gear which is where I wanted "more gear" most of the time.

If I had a manual trans, I wouldn't go with less than 4.56 for 35's. I'm not sure if the need for 4.88s is as compelling on the JL as it was on the JK. If I was paying to regear, I wouldn't go with 4.10s. But if I had 4.10s from the factory, I'd at least drive it for a while on 35's before deciding to regear.

 

[omnitonic]

Looks like the 4.56s on 35s will end up pretty close to a stock Rubicon with 4.10s, maybe do a test drive of one (if you can find one). Obviously they'll be some more drag from the heavier tires, but may get you a good idea of how it should act with shifting.

That's a good idea, actually. It does come pretty close to the stock Rubicon numbers. I never thought to check that.

 

[Kreepin1]

One thing to keep in mind is that the larger tires are heavier and have more rolling resistance so it's more than just rpm's you are compensating for.

The other thing is that your use case is all about commuting to/from work (which is smart). If you don't really go offroad much, then 4.10's. I have 35x11.5's on my JLR and it is fantastic. If you do want to go offroad then 4.56 is a better choice but you should consider swapping in a set of Rubicon axles instead. You get the gears for commuting and the lockers for offroading.

 

[Some Random Guy]

I only see 6th gear when I'm mostly going downhill, and it probably doesn't even matter whether I'm in 6th or 5th or whatever, because I'm at 0% throttle, and just using the gears to hold me back. I consider the switch from 32s to 35s a significant upgrade, although this is the first time in my life I ever changed a tire size, and maybe I should have held out for the 40s like Lite Brite. All I need is a decently hot girl and a Youtube channel, right?

Are you doing your own gears or paying a shop? If you're paying someone to do the, consider waiting until the new XR package axles become available for order. That might be more prudent if you also need a locker and D44's if you're comfortable bolting them on yourself.
To give you an idea, I'm at 5.13 on 39's. I felt my stock tires with 4.10's were insufficient, I hardly saw 6th gear where I live because I usually only do 74 on the interstate, so I wanted to gear down more. If I lived ANYWHERE I did more than 250 miles per month on the highway, this combo would suck. I also went out once with the 39's on 4.10's and they were actually fine. If someone else was re-gearing, I would not recommend 5.13's unless they're going to over 40" tires or staying off the interstate for 90%+ of their driving. I guess I'm trying to say don't buy the hype of gearing deeper too much. A lot of people I know regret gearing deeper, but are too stubborn to admit it as they scream down the highway getting 9 MPG. If you regularly drive around 80 mph, stick with the 4.10's, even on 35's.

 

[R00STER]

Not to hijack, but with all the discussion of hitting the perfect ratios by messing with the rearend, has anyone consider a bolt on ubder/ overdrive unit like Gear Vendors? Not sure if they make one one jeeps, but it could potentially solve the age old question of fuel economy and off-road capability.

 

[GATORB8]

Not to hijack, but with all the discussion of hitting the perfect ratios by messing with the rearend, has anyone consider a bolt on ubder/ overdrive unit like Gear Vendors? Not sure if they make one one jeeps, but it could potentially solve the age old question of fuel economy and off-road capability.

That'd make for some steep driveshaft angles. It'd have to go between the trans and T-case to be useful in 4wd for increased low range (like a marlin crawler), and if he's already barely holding 6th, more overdrive wouldn't help.

 

[JeepinPete]

Stock Rubicon here. I feel the 4.10's are marginal. 6th only becomes useful 75+mph IMO. As smooth, quiet, and efficient the Pentastar is, running it at 2500-2800 on the highway really has no effect on comfort or mpg in my experience. I should note I have the SOT top which is not quiet on the highway.

Frankly the gear ratios in this trans are to widely spaced for the torque of the engine. I had a '79 Cherokee that I swapped a "wide ratio" NV3500 into. Those ratios were about perfect. Never seemed to lug the engine unless I did something wrong. The D478 has a wider ratio spread, which results in the not torqy engine lugging in a lot of scenarios.

@Some Random Guy , would you attribute your highway performance to gears or tire size?

Pete

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