Help me settle the regear ratio question

[CarbonSteel]

Enlighten me. What is weaker than a 5.38 gear set? Assume I'm running 37's.

The FAD, the axles, the knuckles, the ball joints...

The point is a 5.38 is strong enough and every gear higher (numerically lower) than that is even stronger.

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[omnitonic]

They have videos where they'd put a truck in super low range on a rock garden, let the clutch out and jump out of it and walk beside it.

That's awesome!

 

[omnitonic]

An old CJ3 with a T98/Spicer18 running 4.27 axle gears needed that auxiliary Warn overdrive unit to run over about 45 MPH on the highway.

Warn made overdrives? That's interesting, and cool.

We have an old '70s work truck that screams at highway speeds, but it will hit 60 if you're mean to it. After adding the overdrive, I've managed to coax it up to somewhere a little north of 70. It just doesn't really have enough power to go faster while pushing that overdrive.

Oh well, how many kids can say they helped their dad install a $2,000 overdrive into a $3,000 beater work truck? It was definitely an interesting project.

I just went from 4.10 to 4.56 gears, both on 35's. To me, it was well worth it.

It is at about 4400 or 4450 rpm at 75mph, which is just right.

I hope you mean 2400 and 2450. If not, the GrimmJeeper has got some esplainin' to do.

That was just what I was hoping to find though. You tried it both ways. You didn't use 6th much, and you got better mileage in 5th. That's the very thing I was afraid I might get with 4.10s.

Running in 4th with my current setup puts me pretty close to where I will be in 6th on the new gears and tires. I decided to see what kind of mileage I would get on my commuter loop if I never shifted out of 4th.

I was genuinely surprised. I averaged just over 20 mpg never touching either of the "good gas mileage" gears. I'm going to collect data over a few more loops, but I may end up getting the best mileage by not using 5th or 6th at all. This is another vote in the 4.56 column.

True, just trying to help OP get off the fence. The gear struggle is real, especially in the M/T.

The struggle is real indeed. I have definitely paid attention to the fact that the lower ratios are weaker. Especially on the M186. I think I should still be fine on the 4.56 gears, as they don't have THAT many teeth.

 

[grimmjeeper]

Warn made overdrives? That's interesting, and cool.

Turns out Advance Adapters bought the rights for it and they still produce it.

https://www.advanceadapters.com/pro...n--warn-overdrive-for-dana-18-transfer-cases/

 

[DanW]

Warn made overdrives? That's interesting, and cool.

We have an old '70s work truck that screams at highway speeds, but it will hit 60 if you're mean to it. After adding the overdrive, I've managed to coax it up to somewhere a little north of 70. It just doesn't really have enough power to go faster while pushing that overdrive.

Oh well, how many kids can say they helped their dad install a $2,000 overdrive into a $3,000 beater work truck? It was definitely an interesting project.




I hope you mean 2400 and 2450. If not, the GrimmJeeper has got some esplainin' to do.

That was just what I was hoping to find though. You tried it both ways. You didn't use 6th much, and you got better mileage in 5th. That's the very thing I was afraid I might get with 4.10s.

Running in 4th with my current setup puts me pretty close to where I will be in 6th on the new gears and tires. I decided to see what kind of mileage I would get on my commuter loop if I never shifted out of 4th.

I was genuinely surprised. I averaged just over 20 mpg never touching either of the "good gas mileage" gears. I'm going to collect data over a few more loops, but I may end up getting the best mileage by not using 5th or 6th at all. This is another vote in the 4.56 column.

The struggle is real indeed. I have definitely paid attention to the fact that the lower ratios are weaker. Especially on the M186. I think I should still be fine on the 4.56 gears, as they don't have THAT many teeth.

LOL, yeah, fat fingers on a small keyboard. 2400-2450.

Thanks for the catch. I'll fix that.

 

[omnitonic]

Turns out Advance Adapters bought the rights for it and they still produce it.

https://www.advanceadapters.com/pro...n--warn-overdrive-for-dana-18-transfer-cases/

Right! I never even made the connection, because we were looking at their other overdrive, the Ranger? It seems they no longer make it. All you can buy now is the rebuild kit.

It's kind of neat how my connection to Advance Adapters and to the Borg Warner T-18 connected me to Jeep before I finally got a Jeep. It's like the arrows were pointing me in that direction all along.

 

[plex]

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.

I love that 9 mpg, you know nobody will admit they overgeared, why? That is basically asking men to admit their dick is not as big as they think.

Overgear will not get you to 9 mpg, but most often less desirable than before, especially on highway.

The rule of thumb is tall gear (lower numerical number) helps highway driving, short gear (higher numerical number) helps in town driving and crawling.

So don't be surprised your mpg is not magically higher just because you regeared. Gear can't beat physics. If your Jeep scream down the highway in 2400 rpm, even it is at 8th, you will still get a poor mpg.

 

[grimmjeeper]

I love that 9 mpg, you know nobody will admit they overgeared, why? That is basically asking men to admit their dick is not as big as they think.

Overgear will not get you to 9 mpg, but most often less desirable than before, especially on highway.

The rule of thumb is tall gear (lower numerical number) helps highway driving, short gear (higher numerical number) helps in town driving and crawling.

So don't be surprised your mpg is not magically higher just because you regeared. Gear can't beat physics. If your Jeep scream down the highway in 2400 rpm, even it is at 8th, you will still get a poor mpg.

My 3.6 JK got better mileage at 2,500 RPM than it did with 1,900 RPM.

Every engine has a "sweet spot". Take the RPMs too far in either direction and you lose MPG.

 

[omnitonic]

Now that the tires are in, I finally know what size I actually purchased. The site made this somewhat ambiguous. I ended up with the 315/70R17s. Adjusting my calculator accordingly, and looking at the ratios, it turns out this is going to be an interesting experiment.

New shoes:

First of all, I continued my 4th gear experiment tonight. Today wasn't even an ideal fuel economy day, and I drove pretty aggressively. I never shifted beyond 4th gear, and I got 21.3 mpg for the loop. That's crazy! That's a lot better gas mileage than I was getting using 5th and 6th.

So what makes all of this really interesting is the overdrive effect. Right now on my 32s with my 3.45s, if I'm in one of the overdrive gears, the engine is trying to push a lot of gear with very little torque. I feel the good fuel economy in 4th is due to this being the direct gear, AND due to more torque due to higher RPMs.

If I regear to 4.56, I move the RPMs at 70 to a different gear ratio, an overdrive gear. Overdrive gears are taller, and harder to turn, so my current experiments running in 4th are not that great of a predictor of what I should expect after regearing.

Now the fascinating thing here happens after I update the gear calculator to the final tire size, and try that with the 3.45 gears I currently have.

3.45 gears, new tires, 4th gear at 70 mph: 2359
4.56 gears, new tires, 6th gear at 70 mph: 2245

Given that overdrive gears are taller, and harder to push, and given that higher RPMs deliver more torque, an overdriven 2245 might be harder on the engine than a direct 2359. I might actually get better fuel economy if I continue to accept that 5th and 6th gears are just useless in the Blue Ridge mountains, and don't regear at all.

Of course, this ignores the issues I will have at the other end of the spectrum. If I don't regear, I will be crawling a lot faster down on the low range. An extra 40 feet per minute faster. That's pretty significant.

In any case, putting on the big tires and leaving the stock gears will probably have worse impacts in my off-road experience than on my commuting gas mileage, as long as I continue to drive it like a 4-speed. I should have the option of running on 3.45 gears indefinitely. As far as getting to work and getting the bills paid, it shouldn't massively impact my weekly gasoline budget.

I love nerding out on this stuff!

 

[Kreepin1]

If I regear to 4.56, I move the RPMs at 70 to a different gear ratio, an overdrive gear. Overdrive gears are taller, and harder to turn, so my current experiments running in 4th are not that great of a predictor of what I should expect after regearing.

While it is true that direct drive has less driveline loss (friction) than other gears as do taller (numerically smaller) differential gears this effect is negligible and should be ignored. The other effects @grimmjeeper mentioned are far more important.

You have now taken the biggest unknown out of the equation, the effect of the new tires, and should just pay attention to rpm when comparing the two configurations. So, 4th with 3.45 gears is a really, really good indicator of how 6th with 4.56 gears is going to act. Trust me on this.

 

[omnitonic]

So, 4th with 3.45 gears is a really, really good indicator of how 6th with 4.56 gears is going to act. Trust me on this.

I'll be surprised if it turns out you're wrong, let's put it that way, but I'm still looking forward to gathering empirical data, and observing what actually happens either way.

It has been pretty fun playing with all these numbers. This also goes a long way toward backing up the assertion that the factory 3.45 gears with two overdrives just isn't a good configuration. People were giving me crap for whining about how much I had to downshift, but now I've proven I get demonstrably better fuel economy by just doing the whole damn trip in one gear. I'm still at 21.3 after another full loop. That's insane, and totally unexpected.

 

[DanW]

While it is true that direct drive has less driveline loss (friction) than other gears as do taller (numerically smaller) differential gears this effect is negligible and should be ignored. The other effects @grimmjeeper mentioned are far more important.

You have now taken the biggest unknown out of the equation, the effect of the new tires, and should just pay attention to rpm when comparing the two configurations. So, 4th with 3.45 gears is a really, really good indicator of how 6th with 4.56 gears is going to act. Trust me on this.

That would give you a pretty good idea.

I did a lot of experimenting and it did give a decent feel for what it would be like in 6th with 4.56. But the good news is that after the swap it is even better. Overdrive gears are cut differently and wind up smoother and very slightly more efficient. So I felt it was even better after the swap and in 6th. There are other differences, such as the ratio change when dropping to 5th to pass. Dropping 4th to 3rd (or in my case 5th to 4th) wasn't as direct a comparison as 6th to 5th after the swap.

Best advice I got, though, was from @chevymitchell when he said to stop overthinking it and make a decision. (I was waffling between 4.56 and 4.88 on 35's) He had driven a manual with 4.88's and 37's and said it was perfect. Knowing I am sticking with 35;s, I took his advice and considered his experience and made the call for 4.56. I'm VERY happy with them.

 

[word302]

The thing that none of these gear calculators, and no amount of nerding (unless you have a wind tunnel) can account for, is drag. Rolling mass of the larger tires and more drag from your now taller Jeep. The gearing in the stock MT rubicon is pretty poor. I can’t even imagine the manual with 3.45s. I ran my 6 speed rubicon on 37s around for about a year before changing to 5.13s and couldn’t be happier. Mileage improved, 5th and 6th are back, and it’s just a hell of a lot more fun to drive. There is no way I would even consider 4.10s if I were you. 4.56 if you want the best on road performance. 4.88 if you want the best trail performance. All the calculators be damned.

 

[Kreepin1]

The thing that none of these gear calculators, and no amount of nerding (unless you have a wind tunnel) can account for, is drag. Rolling mass of the larger tires and more drag from your now taller Jeep. The gearing in the stock MT rubicon is pretty poor. I can’t even imagine the manual with 3.45s. I ran my 6 speed rubicon on 37s around for about a year before changing to 5.13s and couldn’t be happier. Mileage improved, 5th and 6th are back, and it’s just a hell of a lot more fun to drive. There is no way I would even consider 4.10s if I were you. 4.56 if you want the best on road performance. 4.88 if you want the best trail performance. All the calculators be damned.

He already has the tires so he's comparing apples to apples on his gearing experiments.

 

[word302]

He already has the tires so he's comparing apples to apples on his gearing experiments.

He has the tires but they aren’t on the Jeep yet. The only “experiment” you can do is change the gears out. No calculator is going to tell you the end result.

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