Diesel - 4.56 Gears - 37 Inch Tires

[UTDieselRubi]

4.88s on 37s, I believe it weighs around 5000 loaded, I get 12 mpg and only have gotten the P00af code, I was at the top of hills and mountains and you could hear tye turbo singing , clear it and go with No more problems

Know what your RPMs sit at when going up hill?

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[ericw.]

I'm glad I could help get some info out. I know even though these diesels like to get too hot, no way I'd like towing with the 3.6. Just a test drive in one, it felt incredibly underpowered.

Not only underpowered but it also runs hot. Climbing hills is a balancing act between speed/rpm and engine coolant temp... even on small hills.

I did everything I could.. Mishimoto radiator, replaced thermostat multiple times, ... all the things. I even replaced the motor with a new one from Jasper's Engines. It just doesn't like hauling a bunch of weight.

 

[tj3089]

Not only underpowered but it also runs hot. Climbing hills is a balancing act between speed/rpm and engine coolant temp... even on small hills.

I did everything I could.. Mishimoto radiator, replaced thermostat multiple times, ... all the things. I even replaced the motor with a new one from Jasper's Engines. It just doesn't like hauling a bunch of weight.

You are referring to the old 3.6 gasser you had? Just to clarify for the people in the back.

 

[ericw.]

You are referring to the old 3.6 gasser you had? Just to clarify for the people in the back.

That's correct, the JK 3.6 with a 5 speed automatic.

 

[LOL WUT]

Just dropped mine off this morning for 4.56s. I found that with the new 37s I wasn’t getting into 8th gear on a flat roads unloaded unless there was absolutely 0 wind or a tail wind. While towing our SXS and gear (4500ish lbs) we were holding 6th and occasionally 7th gear on flat roads at 65mph. I’m very excited to get it back and see how it does towing now.

For reference, my old 37s were BFG K02s which notoriously run small and therefore weigh less. With the K02s I would occasionally hold 8th gear on flat roads but still not as often as I’d like, lots of hunting back and forth. The new 37s are Firestone destination MTs and are significantly heavier and larger. I am also running steel bumpers and a 12k Warn winch.

I’ll do my best to get some updates in here once I get the jeep back and some miles on it for those who are looking for more info. I’m towing the sxs about 7hrs away to Windrock in March which will be the first nice test.

 

[LOL WUT]

First update for my 4.56/37” setup. Picked it up this afternoon and everything seems great. Updated the PCM to account for the 4.56s and off I went. Definitely a noticeable difference in terms of low end grunt. Gears are silent as they should be. I jumped on the highway for 2 miles and was pleased to see 8th gear easily. Can’t wait to put the trailer behind it and see how it does.

 

[LOL WUT]

Bumping this back up after my first time
towing with the 4.56s in the diesel.

We had about 4,400 lbs hooked up behind the Gladiator which was a 22’ dual axle trailer with our 4-seater side by side and gear. In total we towed for about 250 miles. We battled some extreme headwinds on the way to our destination (40+mph) and the jeep held 7th gear for a good portion of the drive, rarely getting into 8th except for the brief moments when the wind subsided, not great conditions for the first tow.

However, on our way back home, conditions were much better with little to no wind. Set the cruise at 72mph (2k rpm) and held 8th gear all day long without issue. I forgot to hand calculate my MPG so I don’t have a figure to share.

Overall the jeep pulls the weight significantly better with the 4.56s as expected. Very happy with the change.

I also added rear air bags to help with any bouncing/swaying while towing and they also helped immensely, highly recommend them for anyone who tows. For reference, I have a 2” lift and went with the air lift 60818HD kit which is for a Dodge Ram 1500, this kit fits the larger springs from the lift really well. Inflated to 30psi the jeep felt rock solid.

 

[jc99]

Throwing out another opinion data point for anyone doing their research in that search bar above like I was for some time.

Finally pulled the trigger on gears and traveled all the way to Cali to save 1500$, (why are utah shops so expensive!?). went to 4.56s and absolutely love them! The truck feels better than stock on 33s now. This ratio has my vote for 37s. No longer do I have to put my 33s back on for the daily drives and tow trips, feeling inadequate about my tire size I can now rock only 37s like a true jeep Baja Boss MTs for the trials and toyo at3s for the road, 24MPG at 75mph on Utah I15 average, with a 2" lift to boot. 100% would do again.

@UTDieselRubi What was your mileage before the 4.56?

 

[BlueScapegoat]

Since this is freshly bumped, just wanted to add I recently regeared my diesel to 4.88s with 40s and it's nice. (20 rpm lower at 70 mph than 4.56 and 37s) It feels like it's a thousand pounds lighter. Up and down hill on the highway, holds 8th gear. MPG is very similar so far, which I kind of expected around town, but I'm still breaking them in. I am running a trans tune but it behaves way better than it did with the 3.73s and 40s.

It feels super gross on a cold day until the trans heats up though, but that's just auto trans stuff.

 

[caryt]

Trevor, rpm at 70? MPG's? What tires and wheels?
Thanks

 

[UTDieselRubi]

@UTDieselRubi What was your mileage before the 4.56?

Averaged closer to 20mpg with 37s on the same freeway drives.

 

[BlueScapegoat]

Trevor, rpm at 70? MPG's? What tires and wheels?
Thanks

70 mph on 4.88s and 40s is 1836 rpm.
4.56s and 37s is 1855 rpm.
Stock gears/tires is 1717 rpm.

great gear calculator

I can't really give you any sort of useful mpg figures at this time or ever really, I think it's sitting at about 18 mpg right now but I've just been doing the 12 minute work commute with some idle time to warm up a bit as well. It's pretty much the same it's been, in that use case.

Best mileage I ever saw on the 40s and 3.73s was 26 mpg driving across rural Missouri ~55 mph. There are just too many variables. It's also deleted and tuned, I run it in a mildly aggressive map.

40" STT Pros, 17" AEV Salta XRs with spacers. Favorite MTs I've ever owned, and I've gone through a lot.

 

[ericw.]

40" STT Pros, 17" AEV Salta XRs with spacers. Favorite MTs I've ever owned, and I've gone through a lot.

I loved the STT Pro's. They perform excellent in all conditions *for an MT* but I only got about 25k miles out of them before they showed significant wear.

If that doesn't bother someone, they're an excellent tire. I'm testing out Nitto's trail grappler now. People usually talk about a longer lifespan with the nitto, they just rate lower in the winter driving categories.

 

[caryt]

Think you will like the Trail Grapplers, SST's worked great off-road but not as good on road plus lousy to balance TG's balance great.

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