(updated) First impressions of 35's on my Eco Diesel

[TheSolarWizard]

The aerodynamic profile of a JT is completely different than a ram.


@WXman what does your ram get at 68 mph and what size tires are on it?
That seems to be the speed above which the JT mpg turns down. I don’t want to drive much further than 50 miles at the same speed before the motor is broken in but I’m happy to report that and hand calculate it. My truck will have the 245/75/17s on it when I drive off

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[WXman]

The aerodynamic profile of a JT is completely different than a ram.


@WXman what does your ram get at 68 mph and what size tires are on it?
That seems to be the speed above which the JT mpg turns down. I don’t want to drive much further than 50 miles at the same speed before the motor is broken in but I’m happy to report that and hand calculate it. My truck will have the 245/75/17s on it when I drive off

The aero profile only matters in terms of drag coefficient, and the drag coefficient between the two trucks is not different enough to matter. Plus, the JT has ESS where the DT doesn't, which in terms of MPG would offset any assumed additional drag with the JT body style.

Edit: JL Wrangler was published as having a 0.454 coefficient of drag. The JT is likely slightly different, but not by much. The DT Ram was published with a 0.357 coefficient of drag, BUT, that's a 2WD Quad Cab model which is a very rare configuration. A 4WD Crew Cab with offroad suspension is likely quite a bit higher.

I am currently running 33x12.50 tires. If I set the cruise at 68 MPH and take a long trip down these Kentucky interstates I will see 21-23 MPG depending upon weather. I have noticed that dropping my speed to 60 makes a huge difference, as expected.

I've had a zillion gas powered Jeeps and trucks. I've installed various gear ratios, tires, etc. I've done all sorts of mods to try and free up horsepower and MPG. Electric fans in place of clutched fans, aero pieces, etc. What I've found over the last 25 years is that gas engines are a lot less affected by RPM than diesel engines. I can run 55, 60, 70, whatever in a gas truck and it'll produce very similar MPG. In a diesel truck, speed makes a HUGE difference, and that's because engine RPM makes a HUGE difference.

That's why I've become such an advocate for leaving the gearing in the axles alone, especially with a diesel.

 

[stickshifter]

Negative. Let me explain more clearly what I meant:

The diesel has a super narrow power band. It makes peak torque at 1,600 RPM. By 4,000 RPM it's out of juice. Totally different animal than a gas engine. Completely different.

On a gas engine, higher RPM actually puts you in the "meat and potatoes" of the power band, so, gearing the axles down to raise the RPM up can be a benefit sometimes. Not always but sometimes. And that's why guys traditionally have always re-geared Jeeps. Jeeps needed more RPM to start with, so adding larger tires made a bad situation worse. Re-gearing was necessary.

On a diesel, the power band is really narrow and the sweet spot is right off idle. So your goal is to keep RPM as low as possible. Current Jeeps are coming from the factory over-geared. 3.73 is overkill. My truck has 3.92 which is overkill. So what we need is to either gear the axles UP (numerically lower), or, add taller tires. By adding taller tires you're effectively doing the same thing, i.e., dropping RPM. And that's what you want. Lower RPM.

We can see a perfect example of this in the Ram with the same engine and transmission as the Jeep. The guys on the Ram side of the forum who have 3.21 axle gears are seeing about 20% better fuel economy on average, compared to the guys with identical trucks running 3.92 gears. It's all because on a diesel the opposite is true; you want LESS RPM not more.

Ahhh, I think I get your point. Thanks! I'm going to put it in my own words - and if you don't mind - correct me if I'm wrong:

(1) The stock ecodiesel gearing (3.73) is not the correct gearing for a stock tire (when you say stock tire do you mean the Rubicon 33-inch tire, of the other models which come with what - like a 29 or 30?).

(2) Because stock gearing is over-geared, moving up in tire size (to a 35?) actually results in the correct gearing (i.e. gearing that better matches the diesel engine's power band).

According to your calculations, would the following make sense in the JT Diesel?

3.73 - 35 inch tire
4.10 - 37 inch tire
4.56 - 40 inch tire
4.88 - 42 inch tire

Thanks!

 

[OLAF 4X4]

Just picked up my diesel Sport S on Thursday after getting a Mopar 2" lift with BFG KO2 37's on Mopar beadlock wheels (no pics yet).

My initial observation pulling out from the dealership was a nominal loss in perceived power and a little more time through 1st and 2nd gear. After only 3 days driving her I'm already used to it.

On the fuel mileage side, although she only had 538 miles when the lift went in ... so far I'm getting BETTER fuel mileage than with the stock tires (31.5")! Now granted I was very pedal heavy during that first 500 miles, it's been a while since my last turbo vehicle so I needed to make sure that turbo worked ok

8th gear doesn't come in until around 60-65 mph depending on the grade but it holds 8th just fine up through 75 mph. Should be interesting to see how it tows my 3500 lb camper this summer (at Denver elevation to boot).

How long did you have to wait on your lift? My dealership is telling me it is on back order. I ordered mine at the beginning of February and it’s still not in. Was hoping to have it installed before I took deliver of my JTRD. I’ve had it for Going on 2 weeks now. 37’s and Mopar beadlocks are sitting in the garage very lonely waiting on that 2” mopar diesel lift.

 

[JT LIFE]

I felt your pain, ordered up the lift and it took a whole month to arrive and the tires just sat there mocking me.......
But it will be worth the wait!

 

[BEERviper]

How long did you have to wait on your lift? My dealership is telling me it is on back order. I ordered mine at the beginning of February and it’s still not in. Was hoping to have it installed before I took deliver of my JTRD. I’ve had it for Going on 2 weeks now. 37’s and Mopar beadlocks are sitting in the garage very lonely waiting on that 2” mopar diesel lift.

I called the parts department to order everything on Jan 15th. That was the Mopar 2" diesel suspension lift, 37" KO2's, Mopar beadlocks, and Rockslide Engineering step sliders.

Everything but the lift was in within 3 biz days. I got the call on Feb 3rd that the lift was finally delivered so less than 3 weeks from order to delivery.

I feel like more and more of these are being ordered and they are having trouble keeping up with demand which is pushing people towards other vendors.

 

[FutureOdin]

Ahhh, I think I get your point. Thanks! I'm going to put it in my own words - and if you don't mind - correct me if I'm wrong:

(1) The stock ecodiesel gearing (3.73) is not the correct gearing for a stock tire (when you say stock tire do you mean the Rubicon 33-inch tire, of the other models which come with what - like a 29 or 30?).

(2) Because stock gearing is over-geared, moving up in tire size (to a 35?) actually results in the correct gearing (i.e. gearing that better matches the diesel engine's power band).

According to your calculations, would the following make sense in the JT Diesel?

3.73 - 35 inch tire
4.10 - 37 inch tire
4.56 - 40 inch tire
4.88 - 42 inch tire

Thanks!

And, theoretically, actually moving to a 35 inch tire with stock 3.73 gears would increase MPG? no?

 

[DesertDog]

I've been running 35s on my EcoDiesel since I picked it up on Oct. 23 (BFG KO2s, 35x12.5x17). Did about 800 miles that way, then added the 2" Mopar lift last week (and got the recalibration for tire size). Added another 500 miles since then.

Definitely better since the recalibration. I managed almost 24 mpg on a 400 mile highway trip. Seems to do worse on MPG over 60, and hits a sweet spot from 40-55 where I was topping out over 24. overall milage is around 22, about inline with everyone else 2 mpg drop. There was a little off-line hesitation at first, but I've either gotten used to it or the recalibration worked that out a bit. Much less noticeable now. I didn't see any top-gear hunting on the highway, and a good part of the trip was crossing some East Coast mountains, so elevations shifts of up to 1000 feet or so. Seemed to hold 8th pretty well.

Overall ride, even with the lift, is excellent. Surprisingly smooth. Very happy with the look and feel.

I just ordered an eco-diesel amazingly similar to yours. Stingray Rubicon black hardtop, rollup bed cover… etc. I’m planning the exact same KO2s when it arrives. I see you are using the stock wheels. The tire shops I’m speaking with tell me I have to get wider wheels for the 12.5s. Did you lift because of rubbing? I have not made that decision yet. Thanks. Lance.

 

[2TH MVR]

What's everyone paying for dealership labor on the 2 inch diesel mopar lift? Should be the same for every dealership. Right?

 

[CerOf]

I just ordered an eco-diesel amazingly similar to yours. Stingray Rubicon black hardtop, rollup bed cover… etc. I’m planning the exact same KO2s when it arrives. I see you are using the stock wheels. The tire shops I’m speaking with tell me I have to get wider wheels for the 12.5s. Did you lift because of rubbing? I have not made that decision yet. Thanks. Lance.

Stock wheels are just fine. My 35x12.5 diratracs were purchased and installed by discount tire. I have stock suspension in my rubicon.

 

[John in the Woods]

I just ordered an eco-diesel amazingly similar to yours. Stingray Rubicon black hardtop, rollup bed cover… etc. I’m planning the exact same KO2s when it arrives. I see you are using the stock wheels. The tire shops I’m speaking with tell me I have to get wider wheels for the 12.5s. Did you lift because of rubbing? I have not made that decision yet. Thanks. Lance.

Thanks my friend, and I can tell you you're going to love the new rig. I'm hitting 24+ mpg on the highway with the 35s and the 2-inch lift, and averaging just about 20 overall. Great ride from the Mopar lift around town. I added the lift and the 35s just to address the breakover issue, and add a little more ground clearance. I generally try to avoid upgrades until use proves I need them, but I read enough here and elsewhere to suggest it might be a smart move -- and doing them out of the gate let me build it all into the purchase.

I will admit that I've been thinking about new rims (eying the AEV Pintler wheel made for the JL/JT). Those 12.5s seem to crown and curve under around the edges, even if I drop pressure to the low 30s. I'll see how it feels on the trail at lower psi, but it could be a little wider rim in the end make sense.

Love these tires, by the way. Pulled through a very heavy winter of snow, no issues. Can't wait to start given them some trails and rocks to chew on.

 

[BEERviper]

What's everyone paying for dealership labor on the 2 inch diesel mopar lift? Should be the same for every dealership. Right?

Labor at my local dealer in Longmont, CO was $440 for the Mopar 2" lift.

 

[Up2spec]

I went with 35s on my overland diesel. I'm glad I read about people feeling the difference with recalibrating the tire size. I noticed the difference right away after recalibrating. It felt sluggish before and now it feels like it can breath.

 

[DesertDog]

Sorry if this was already answered but does anyone know the cost of having the dealership reprogram for tire size? The idea of having to remove a Tazer every time you visit the dealership sounds like a pain.

 

[mrmo]

I don’t get your argument. You say diesels have a narrow power band, so don’t regear. Isn’t that even more of a reason to regear if you put on bigger tires? The larger tires will throw off the factory designed gear ratio, so the narrow power band of the diesel makes it even more important to get back to the intended ratio, hence, you need to regear. If the power band was super broad, regearing would be less crucial.

I agree...my Powerstroke has 4.30 gears w approx 34" tires, cruises at 2400 in overdrive, has a shit load of torque and power. The gearing comes into play in the lower end getting a load up and moving, then the overdrive calms it down. 37's w 4.10 would be excellent

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