Regear with 38s

Brian

Jeep Fanatic
#21
My sons Jeep is a 14 JK 3.6l auto trans with 5.13 gears and BFG 37” tires. It’s way too much IMO. Maybe good for more mountainous terrain but here at 70mph it’s turning almost 3k RPM. Swapping them out for 4.56 gears in the next few weeks.
Serously, what is the difference between running 2700 at 70 or 3000 at 70? Please dont tell me fuel mileage.... because fuel mileage is going to be more based off engine load than RPM. You will most likely get worse fuel mileage with 4.56's at 70 than 5.13s..... IMHO, from experience, your making a mistake changing gears.
 

SSinGA

Jeep Fanatic
#23
Serously, what is the difference between running 2700 at 70 or 3000 at 70? Please dont tell me fuel mileage.... because fuel mileage is going to be more based off engine load than RPM. You will most likely get worse fuel mileage with 4.56's at 70 than 5.13s..... IMHO, from experience, your making a mistake changing gears.
I guess we’ll see soon.
 

Pthorpe84

Moderator
Staff member
#24
Serously, what is the difference between running 2700 at 70 or 3000 at 70? Please dont tell me fuel mileage.... because fuel mileage is going to be more based off engine load than RPM. You will most likely get worse fuel mileage with 4.56's at 70 than 5.13s..... IMHO, from experience, your making a mistake changing gears.
I think you will see a small increase in the mpg.




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Last edited:

Brian

Jeep Fanatic
#25
I think you will see a small increase in the mpg but not much. But where it will help is that you can use O/D more instead of always using 5th gear. Plus the ability to climb a hill at highway speeds Vs losing speed and down shifting.


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He is going from 5.13 to 4.56. Should be the exact opposite of what you said. Come on Bear Poker.

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Pthorpe84

Moderator
Staff member
#26
Yeah I I just edited the post. I’m a dumb ass bear poker. But if it’s highway use mainly he should be good to go. Maybe it’s really flat where he is vs hills


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#27
Serously, what is the difference between running 2700 at 70 or 3000 at 70? Please dont tell me fuel mileage.... because fuel mileage is going to be more based off engine load than RPM. You will most likely get worse fuel mileage with 4.56's at 70 than 5.13s..... IMHO, from experience, your making a mistake changing gears.
Engine heat? And maybe not as much in OD, but when downshifting for hills?
 

Brian

Jeep Fanatic
#28
Engine heat? And maybe not as much in OD, but when downshifting for hills?
Engine heat, in this case is also based on load. When downshifting down hill, higher rpm would be best. No matter how you slice it, changing from 5.13 to 4.56 is not a smart more over 250 rpms.

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#29
Engine heat, in this case is also based on load. When downshifting down hill, higher rpm would be best. No matter how you slice it, changing from 5.13 to 4.56 is not a smart more over 250 rpms.

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Unless your high RPMs are putting you into an inefficient band in the engine, right? Which, again, not at 3000 RPMs, but how many RPMs are you at in lower gears going UP a steep pass at 75mph? In my experience this is where temps spike.
 

Brian

Jeep Fanatic
#30
Unless your high RPMs are putting you into an inefficient band in the engine, right? Which, again, not at 3000 RPMs, but how many RPMs are you at in lower gears going UP a steep pass at 75mph? In my experience this is where temps spike.
Your temps would be spiking because of the load on the engine not because of the RPM

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Brian

Jeep Fanatic
#32
Increasing due to increasing load I'll give you. But in my experience a spike comes at certain shift points.
What year Jeep are we talking about because these newer engines with the programs in fuel injection can run at 5000 RPMs all day long without overheating

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Brian

Jeep Fanatic
#34
2012+ JKs. Lots of people seeing 230+ easily.
I can imagine there are tons of people overheating because not many actually re gear like they should to keep the load the same on the engine. Let's face it in this thread we have a guy who is geared properly and is going to gear himself all the way back to close to a stock Rubicon for going up in tires almost 5 inches. Purely incorrect

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#35
I can imagine there are tons of people overheating because not many actually read year like they should to keep the load the same on the engine. Let's face it in this thread we have a guy who is geared properly and is going to gear himself all the way back to close to a stock Rubicon for going up in tires almost 5 in purely incorrect

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Fair point in general. But if you run the numbers, 4.56 for 37s is pretty close in gearing, through all the gears, to a stock auto with 32s and 3.73 gears. Actually a little lower, almost like with 4.10s. And remember the 4.10s aren't even considered necessary on a Rubicon, they're optional for the auto. (Makes me think Chrysler doesn't think they match up well with the 5 speed).

Look at the numbers below, which make me think a lot of people are actually OVERgearing compared to stock...

This is 4.56 with 37s vs 3.73 with 32s.


Then 5.13 with 37s vs 3.73 with 32.
 

Brian

Jeep Fanatic
#37
Fair point in general. But if you run the numbers, 4.56 for 37s is pretty close in gearing, through all the gears, to a stock auto with 32s and 3.73 gears. Actually a little lower, almost like with 4.10s. And remember the 4.10s aren't even considered necessary on a Rubicon, they're optional for the auto. (Makes me think Chrysler doesn't think they match up well with the 5 speed).

Look at the numbers below, which make me think a lot of people are actually OVERgearing compared to stock...

This is 4.56 with 37s vs 3.73 with 32s.


Then 5.13 with 37s vs 3.73 with 32.
You're comparing stock numbers with smaller tires in comparing RPM and I'll think about the load with tires that way two to three times the amount as to what stock ones do along with rims that way two to three times the amount so you're rotational mass increases exponentially. That my friend is why you need higher gears than stock along with better brakes

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SSinGA

Jeep Fanatic
#38
Y’all, before you blow a gasket over this gearing thing there are a few facts you need to know.

Kid will be living in south Alabama (flat).

Home will be 5 hours north (still flat)

Off road use will be limited for a quite a few years as he will be focusing on college, being a collegiate athlete and med school.

BFG KO2 37s are more like large 35s.

He’ll be fine on the 4.56 gears until he is employed and can make this jeep a dedicated trail rig.
 
#39
You're comparing stock numbers with smaller tires in comparing RPM and I'll think about the load with tires that way two to three times the amount as to what stock ones do along with rims that way two to three times the amount so you're rotational mass increases exponentially. That my friend is why you need higher gears than stock along with better brakes

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I follow you on acceleration/brakes to a degree, but how much of an effect does that rotational mass have on maintaining a constant speed? Esp with all the other weight added to the jeep.
 

Brian

Jeep Fanatic
#40
I follow you on acceleration/brakes to a degree, but how much of an effect does that rotational mass have on maintaining a constant speed? Esp with all the other weight added to the jeep.
This is pretty basic I mean how easy is it to keep a bicycle wheel turning at 35 miles an hour by hand or how easy is it to keep a 200lb 37 inch tire turning by hand there's obviously going to be a lot more load on a much heavier and wider Tire. We're not even getting into the fact that they stick out farther from the vehicle.

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