Help me understand re-gearing

TJim

TJim

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Hey everyone.

So this is a completely noob thread. I am starting to figure the most things about my build out. I am just having some more questions about re-gearing.

I have a 2004 2.4L TJ, with NV1550 and 4.10 gears. My TJ came stock with 215/75r15 tires, and now I run 235/70r16.

My goal is to run 31x10.5r15 tires, 15x8 wheels with 3.75 backspace. Then, a 2 inch suspension lift kit.

1) So, I would like someone to explain as plain as possible what running 31" with the stock 4.10 ratio means?
For example: less RPM at 70mph with 31" than 29"?

2) Which would be the difference, if I install in the future (when the budget allows), re-gearing from 4.10 to 4.56 (31" tires).
For example: less RPM at 70mph with 4.10 than 4.56?

Will I have (theoretically) better mpg when I re-gear from 4.10 to 4.56, while running 31" tires?
Which other would be the major differences, when
a) I change from 29" tire to 31" tire with the 4.10 gears?
b) I change from 4.10 gears to 4.56 gears running 31" tires?

Please, help me understand what to expect, and the real reasons behind re-gearing.

*I should note that this is my daily driver, but I also enjoy off road trips, and I would like to make it as capable as a 2,4L could be, without suffering every day on road. To find a balance between on-road and off-road.
 
Use the above gear calculator.

Having said that trust me when I tell you if you're planning to change the axle gears and not the engine/tranny...
You definitely want to jump to at least a 4.88:1 and if you see 33's in your future 5.13's.

The wildcard in this is while the taller tires turn less times per distance traveled...they're also heavier.
In other words it pays to over gear...especially if keeping the 4 cylinder.

I'd say from reading your posts you're going to get addicted to this hobby quickly and I see 32's and or 33's in your future (after 31's)

Your target should be 4.88's and forget the 4.56. Even if staying with a 31" tire...4.88 is your gear. Not enough of a bump just going to a 4.56

Thats my opinion on that
 
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Rubicon88 said:
Use the above gear calculator.

Having said that trust me when I tell you if you're planning to change the axle gears and not the engine/tranny...
You definitely want to jump to at least a 4.88:1 and if you see 33's in your future 5.13's.

The wildcard in this is while the taller tires turn less times per distance traveled...they're also heavier.
In other words it pays to over gear...especially if keeping the 4 cylinder.

I'd say from reading your posts you're going to get addicted to this hobby quickly and I see 32's and or 33's in your future (after 31's)

Your target should be 4.88's and forget the 4.56. Even if staying with a 31" tire...4.88 is your gear. Not enough of a bump just going to a 4.56

Thats my opinion on that
Click to expand...
Thanks for the response!

As you said, I am sure that I will get addicted to this hobby, if I am not already. I loved Jeeps since I was really young, and now at 25 I have my own TJ to use as a platform for my first build.

So, for the foreseeable future, there is not an engine swap for sure. Maybe in 10 years or so! I have a NV3550 sitting in the garage, but I am not sure if there is any point of swapping right now, since my NV1500 works perfectly atm.

So, back to re-gearing. How 4.88 would work with 31" daily driving it? (lets say for the sake of the conversation I am not going to run more than 31" in this vehicle)

Also, I think (using some quick calculations) that my new tires/rims will not be heavier than the new ones, or at least there will not be a big difference.
Now: 16" Moab + 29" Goodyear Wrangler A/T Load Range E = 26 pounds + 34 pounds = 60 pounds.
Total Weight: 60 pounds x 4 tires = 240 pounds.

New setup: 15" Pro Comp 7069 + 31" BF KO2s Load Range C = 16-19 pounds + 44 pounds = 60-63 pounds.
Total Weight: 60-63 pounds x 4 tires = 240-252 pounds.
 
TJim said:
the real reasons behind re-gearing
Click to expand...
Forget better mpg. It's not really going to change that.

The real reason for re-gearing is to accommodate for the larger turning radius and weight of the tire. Without it the Jeep will be sluggish. With it the Jeep will have power.
 
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TJim said:
Thanks for the response!

As you said, I am sure that I will get addicted to this hobby, if I am not already. I loved Jeeps since I was really young, and now at 25 I have my own TJ to use as a platform for my first build.

So, for the foreseeable future, there is not an engine swap for sure. Maybe in 10 years or so! I have a NV3550 sitting in the garage, but I am not sure if there is any point of swapping right now, since my NV1500 works perfectly atm.

So, back to re-gearing. How 4.88 would work with 31" daily driving it? (lets say for the sake of the conversation I am not going to run more than 31" in this vehicle)

Also, I think (using some quick calculations) that my new tires/rims will not be heavier than the new ones, or at least there will not be a big difference.
Now: 16" Moab + 29" Goodyear Wrangler A/T Load Range E = 26 pounds + 34 pounds = 60 pounds.
Total Weight: 60 pounds x 4 tires = 240 pounds.

New setup: 15" Pro Comp 7069 + 31" BF KO2s Load Range C = 16-19 pounds + 44 pounds = 60-63 pounds.
Total Weight: 60-63 pounds x 4 tires = 240-252 pounds.
Click to expand...
The moment you go to a 31 you will see the difference.
I'm not saying you can't drive it but you will definitely lose 5th gear on the slightest of hills and you will only pass mile markers and sight seekers.

The little 4 has no power and really needs all the help it can get. Changing ring and pinion can be expensive if you don't do it yourself and you only want to do it once.
At a 4.88 gear you will be a littke rpm heavy with a 31...but with the 4 that is a good thing. Trust me. It also allows you a little buffer should you want 32's or 33's.

4.56 won't really do much for you as you're already using 4.10's.
 
JMT said:
Forget better mpg. It's not really going to change that.
Click to expand...
Regearing absolutely can improve mpg. My TJ was lugging with engine rpms that were too low for its tire size and transmission type. It consistently received in the upper-11mpg range... never 12.0 or better mpg. I regeared from 4.88 to 5.38 and with the engine now operating more efficiently at slightly higher rpms and no longer lugging, my mpg jumped up to 14.6 mpg on the highway. It is now consistently over 14 mpg on the highway which is not bad for a rock crawler TJ running 35" tires and the 42RLE automatic transmission.
 
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TJim said:
Hey everyone.

So this is a completely noob thread. I am starting to figure the most things about my build out. I am just having some more questions about re-gearing.

I have a 2004 2.4L TJ, with NV1550 and 4.10 gears. My TJ came stock with 215/75r15 tires, and now I run 235/70r16.

My goal is to run 31x10.5r15 tires, 15x8 wheels with 3.75 backspace. Then, a 2 inch suspension lift kit.

1) So, I would like someone to explain as plain as possible what running 31" with the stock 4.10 ratio means?
For example: less RPM at 70mph with 31" than 29"?

2) Which would be the difference, if I install in the future (when the budget allows), re-gearing from 4.10 to 4.56 (31" tires).
For example: less RPM at 70mph with 4.10 than 4.56?

Will I have (theoretically) better mpg when I re-gear from 4.10 to 4.56, while running 31" tires?
Which other would be the major differences, when
a) I change from 29" tire to 31" tire with the 4.10 gears?
b) I change from 4.10 gears to 4.56 gears running 31" tires?

Please, help me understand what to expect, and the real reasons behind re-gearing.
Click to expand...
The simple answer is that when you regear to a lower axle ratio, it raises the engine rpms. More engine rpms means more power.

Larger diameter tires need more power to turn since their larger radius gives them more leverage against the engine. Raising the engine rpms by regearing the axles to that lower ratio allows it to generate more power to turn the larger diameter tires.

So yes, you'd have less rpms at 70 mph with 4.10 vs. 4.56.

And so there's no confusion... 4.88 is a lower ratio than 4.56 is. 3.07 is a much higher ratio than 4.10 is. Going to a lower ratio would be, for example, like going from 4.10 to 4.88. So when we say regear to a lower ratio, it means you're going to gain higher engine rpms at any given speed.

No, higher engine rpms does not necessarily mean excessively high rpms. You usually achieve slightly higher rpms when regearing to a lower ratio. When I regeared from 4.88 to 5.38 a couple years ago, my rpms went up a couple hundred rpms at 70 mph. The engine went from having too low of an rpm at 70 mph which meant it wasn't producing enough power to easily turn my 35" tires. Even going up a very minor slope meant I had to drive it very aggressively by downshifting far sooner and more often than would be necessary with an appropriately geared Jeep would. Even 5.38 is not 'quite' enough for my 35" tires and 42RLE transmission but 5.38 is as low of a ratio as my axles can handle without changing out my lockers to a different type which would be an expensive thing to do.

Don't be afraid to regear, it's a very commonly done thing. It's commonly done because it works and the results are very satisfying. And regeared to the proper ratio, your highway rpms will be perfect. When we recommend ratios here, we are recommending ratios that are suitable for street use... not just for offroad use.
 
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Jerry Bransford said:
Regearing absolutely can improve mpg. My TJ was lugging with engine rpms that were too low for its tire size and transmission type. It consistently received in the upper-11mpg range... never 12.0 or better mpg. I regeared from 4.88 to 5.38 and with the engine now operating more efficiently at slightly higher rpms and no longer lugging, my mpg jumped up to 14.6 mpg on the highway. It is now consistently over 14 mpg on the highway which is not bad for a rock crawler TJ running 35" tires and the 42RLE automatic transmission.
Click to expand...
I know yours did, it's just not a hard and fast rule. It can, but generally I don't see that regearing improves or unimproves gas mileage. Not a reason I would tell someone to regear! That might give them false expectations!
 
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JMT said:
I know yours did, it's just not a hard and fast rule. It can, but generally I don't see that regearing improves or unimproves gas mileage.
Click to expand...
If the engine is lugging badly as mine was then regearing to a more appropriate ratio to stop the lugging will improve mpg. If it's not lugging it probably won't.
 
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Following on from this, how can I find out what the gearing ratio of my TJ is? I don’t have a full service history and it’s had multiple owners. From what I know, the original owner had it lifted then the guy I bought it from dropped it back down to stock but he doesn’t know anything about the hearing either. It’s currently running 245 KO2s.

FWIW I’m putting new springs with a 1.5” soon.
 
JM_TJ said:
Following on from this, how can I find out what the gearing ratio of my TJ is? I don’t have a full service history and it’s had multiple owners. From what I know, the original owner had it lifted then the guy I bought it from dropped it back down to stock but he doesn’t know anything about the hearing either. It’s currently running 245 KO2s.

FWIW I’m putting new springs with a 1.5” soon.
Click to expand...
Type your VIN # in here. Everything about the original build, including gear ratio will be listed.

https://fcacommunity.force.com/RAM/s/equipment-listing
 
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JM_TJ said:
Following on from this, how can I find out what the gearing ratio of my TJ is? I don’t have a full service history and it’s had multiple owners. From what I know, the original owner had it lifted then the guy I bought it from dropped it back down to stock but he doesn’t know anything about the hearing either. It’s currently running 245 KO2s.

FWIW I’m putting new springs with a 1.5” soon.
Click to expand...
It may not still be there but the factory installed a ratio tag at the 9 o'clock position of both differential covers.

This tag is indicating the axle has 3.73 gears inside.

Axle Ratio.jpg
 
JMT said:
I know yours did, it's just not a hard and fast rule. It can, but generally I don't see that regearing improves or unimproves gas mileage. Not a reason I would tell someone to regear! That might give them false expectations!
Click to expand...
This right here is spot on.
The Jeep will not realize amazing mpg. Not gonna happen. It's a box. Boxes don't cut through the air so well. Even if all conditions were amazingly rosy what will you see? 1 mpg? 1.73496 mpg increase? A 1.6 mpg decrease?
Well shit...this tank ✌ mpg better. Wow!!! Hold the phone and stop the presses.
Now I get 18.6 mpg rather than 17.4
My goodness with that increase forget rotating in the car pool.
I think I'll just drive everyone to work everyday It's so economical. 😎👍😎
If the auto mfg's only knew what I was up to they'd have aborted all this electric mumbo jumbo many years ago.

It's negligible. It's nothing.
 
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JMT said:
Forget better mpg. It's not really going to change that.

The real reason for re-gearing is to accommodate for the larger turning radius and weight of the tire. Without it the Jeep will be sluggish. With it the Jeep will have power.
Click to expand...
Well said. Good post.

Also noticed you kept asking if it meant less rpm...what it ends up feeling like when you regear is giving it less throttle opening to get it moving. It's feels more responsive to the throttle basically.

All regearing really is is a necassary adjustment to keep the engines power curve and rpm working as designed together when you change tire sizes..for example , the 4:10 gears are there already to compensate for the small displacement engine and on a 4.0 Rubicon they are there because of the 31" tire. The gears connect the power in and power out to the ground.

As far as gas mileage, the only way I know to substantially make a tank of gas last longer in one of these is not to drive it.
 
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AndyG said:
Well said. Good post.

Also noticed you kept asking if it meant less rpm...what it ends up feeling like when you regear is giving it less throttle opening to get it moving. It's feels more responsive to the throttle basically.

All regearing really is is a necassary adjustment to keep the engines power curve and rpm working as designed together when you change tire sizes..for example , the 4:10 gears are there already to compensate for the small displacement engine and on a 4.0 Rubicon they are there because of the 31" tire. The gears connect and the power in and power out to the ground.

As far as gas mileage, the only way I know to substantially make a tank of gas last longer in one of these is not to drive it.
Click to expand...
And to add...I'd bet you an old zip on KC light cover (the smiling one that said daylighter with sunglasses) that the gas mileage might drop a tad. Definitely not going to increase.
Why? In that 4 banger the OP will be like man...it feels like i added another cylinder to this engine and I'm betting the OP puts the hammer down getting up to speed to the likes he's never seen. Maybe even trying to travel back in time 😎

On a serious note I'm glad to see another TJ person building instead of buying. He's asking all the right questions. Now he just needs to pull the trigger and start building. All these texts will make total sense in about 5 minutes once he's gripping it and ripping it.
 
JM_TJ said:
Following on from this, how can I find out what the gearing ratio of my TJ is? I don’t have a full service history and it’s had multiple owners. From what I know, the original owner had it lifted then the guy I bought it from dropped it back down to stock but he doesn’t know anything about the hearing either. It’s currently running 245 KO2s.

FWIW I’m putting new springs with a 1.5” soon.
Click to expand...
If you have a 4 cylinder stick...

You can only have 4.10 gears. *

*unless someone changed it...

Easy...

Jack up rear...turn tire one revolution. Count how many turns said yoke makes to one tire revolution.
If it's real close to 4 complete turns you've got 4.10

If someone changed it (I doubt) and it's like 4.5 turns...you've got 4.56

And so on and so on.

Good luck
 
‘All really good responses here. In an RVing article that I read several years ago it was stated that a gasoline engine achieves its optimal efficiency when running a slightly higher RPM than that which gives you the peak torque. This means that, at highway speeds, you won’t have your foot in the throttle as much if you used the lower gears (4.56, 4.88 & such). If you could see your manifold vacuum, you would notice that it would be higher which means more efficiency & yield higher fuel mileage (not that ANY of us bought Jeeps for gas mileage ☺️).
It does seem counterintuitive that higher RPMs yields better mileage, but it’s true with larger diameter tires. Regearing just brings the RPMs back to the sweet spot.
 
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