Help me understand gearing and re-gearing

finchmeister08

finchmeister08

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Joined
Jun 25, 2019
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25
Location
Eufaula, AL
Here's my setup:

2006 Jeep Wrangler SE
4.0L Engine
42RLE Automatic Transmission
Dana 30 Rear Axle with 3.73 gears
Dana M35 Front Axle with ??? gears (I assume it would be the same 3.73 gears)
4.25" Zone Offroad Combo Lift
33x12.50R15 BFG All Terrain Tires

I got the axle information from a website I found during some research on this website.
Here's the link...

I haven't checked the axles for myself, but I feel confident that the info I gather from that site is correct since everything else was pretty much spot on.

My question is this. Should I upgrade gears? I've read some where people have my same setup, and they're fine. Others say upgrade to 4.56 gears. My Jeep actually accelerates fine around town when I'm only on the road for 10-15 minutes at a time. The "trouble" I notice is when I travel out of town on the highway. It struggles to get up to 65-70 mph when compared to it's stock setup. Uphill climbs bog it down as well. It'll get to those speeds, but to get it there requires the RPMs to be 2500-3000. It'll eventually drop into overdrive when the highway flattens out (RPMs below 2000), but I'm afraid I'm going to burn up my transmission if I continue down this path with it popping in/out of OD. I have the OD switch. Should I just turn it off? Is overdrive a "5th" gear? Or is it the 4th?

I've done the upgrade calculation:

(33" new tire / 30" old tire) * 3.73 upgrade gear ratio = 4.10 new gear ratio
 
You mentioned you have an SE, but a 4.0L engine. Has there been an engine swap? Gearing recommendations are dependent on engine, transmission, and tire size.
 
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finchmeister08 said:
apparently, there was a 4.0 option for the '05-'06 SE models. from what I can tell, everything is stock.

https://en.wikipedia.org/wiki/Jeep_Wrangler_(TJ)
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Interesting, I did not know that.

Either way, with the 42RLE and 33" tires, you'll want to regear to 5.13 (which is the deepest you can go with the front Dana 30). I have a similar setup (42RLE, 33" tires) and recently switched to 5.13, which has been significantly better all around.
 
Deeper (numerically higher) gearing will raise the RPM, which is done to get the engine back into the proper operating range to compensate for larger, heavier tires. The 4.0 engine is most efficient when in the 2600-2800 RPM range. Don't be afraid of "higher" RPMs. Several folks run around 3000 RPM in top gear.

Gear for your top transmission gear at your typical highway cruising speed. From grimmjeeper with 42RLE/32"/70mph:
3.73: 1892 rpm
4.88: 2475 rpm
5.13: 2602 rpm
 
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finchmeister08 said:
Does the 4.88/5.13 make the RPMs run higher? I was under the impression that the lower the gear, the worse the gas mileage is
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Larger tires decrease the rpms. The primary purpose of regearing is to increase the rpms again to account for the larger tire.

Mine is geared for 3k rpm at 75mph. I typically get 13-16mpg. Higher 17s are possible if I try.
 
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I'll be doing a rear axle swap on my 01 from a 35 to a 44. The 44 has 3.73 w/ LSD and disc brakes. I'll be regearing to 4.88s and will eventually be running 35's. The front is a 30 open with 3.07s and will also be regeared.

I've been beating myself up on this, but I can keep the LSD in the rear with the regear without issues....correct?
 
PD11288 said:
I'll be doing a rear axle swap on my 01 from a 35 to a 44. The 44 has 3.73 w/ LSD and disc brakes. I'll be regearing to 4.88s and will eventually be running 35's. The front is a 30 open with 3.07s and will also be regeared.

I've been beating myself up on this, but I can keep the LSD in the rear with the regear without issues....correct?
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You will need a "thick" cut 4.88 to fit on your 3.73 and down carrier.
 
what's been everyone's top end speed on the highway after doing a regear? i remember while it was stock, it didn't quite struggle to get to 70 with 30" tires. but i also noticed that I didn't want to go any faster than that. mainly because its not as a smooth of a ride like a sedan is. now with 33" tires, it can struggle getting to 60-65 (i'm fine with this), but my main concern is it going in/out of overdrive going through inclines and the constant changing of gears. i'm not trying to burn up my automatic transmission.
 
finchmeister08 said:
what's been everyone's top end speed on the highway after doing a regear? i remember while it was stock, it didn't quite struggle to get to 70 with 30" tires. but i also noticed that I didn't want to go any faster than that. mainly because its not as a smooth of a ride like a sedan is. now with 33" tires, it can struggle getting to 60-65 (i'm fine with this), but my main concern is it going in/out of overdrive going through inclines and the constant changing of gears. i'm not trying to burn up my automatic transmission.
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The only solution would be to re gear it correctly. The bigger tires are an added strain on the trans if not re-geared.

Check grimmjeeper for your speed/rpm questions.
 
finchmeister08 said:
what's been everyone's top end speed on the highway after doing a regear? i remember while it was stock, it didn't quite struggle to get to 70 with 30" tires. but i also noticed that I didn't want to go any faster than that. mainly because its not as a smooth of a ride like a sedan is. now with 33" tires, it can struggle getting to 60-65 (i'm fine with this), but my main concern is it going in/out of overdrive going through inclines and the constant changing of gears. i'm not trying to burn up my automatic transmission.
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Post regear, I can do 80+ with 42RLE, 33" tires, and 5.13 gears. RPM is around 3,000 at 80 mph.

I recently took it on a trip from Phoenix to Flagstaff up I-17, which is an up and down highway with several steep climbs. I was impressed at how well it was able to stay in top gear. Only one hill required it to drop down into 3rd to maintain speed (for those in AZ, it was the southbound climb out of Camp Verde). Got the best MPG since I've owned it (16.3).
 
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nk24 said:
Post regear, I can do 80+ with 42RLE, 33" tires, and 5.13 gears. RPM is around 3,000 at 80 mph.

I recently took it on a trip from Phoenix to Flagstaff up I-17, which is an up and down highway with several steep climbs. I was impressed at how well it was able to stay in top gear. Only one hill required it to drop down into 3rd to maintain speed (for those in AZ, it was the southbound climb out of Camp Verde). Got the best MPG since I've owned it (16.3).
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this is what i don't get. in my mind, higher RPMs means more fuel is being burned at a faster rate (with lower gears). which would make your mileage drop. my gas light just came on after a full tank and i've only gone 164 miles on it with it's current set up. a majority of it is around town mileage.
 
finchmeister08 said:
this is what i don't get. in my mind, higher RPMs means more fuel is being burned at a faster rate (with lower gears). which would make your mileage drop. my gas light just came on after a full tank and i've only gone 164 miles on it with it's current set up. a majority of it is around town mileage.
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You want your RPMs in the correct power range. To low and you lug the engine, decreasing power and leading to poor MPGs. You regear to keep the RPMs in the proper range to keep the engine operating effectively.
 
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