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

[coder]

Mopar sells upgraded gears any dealer that says it voids your drivetrain warranty doesn't know what they're talking about:

4.10's factory gears same as a 3.6 Rubicon
4.88 - PN# 77072409 (front)
4.88 - PN# 77072410 (rear)

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[KurtP]

Regear. Add a tune or PC

 

[WXman]

Weight is only part of this equation. The additional leverage you've applied to the driveline is the reason it is sluggish now. Anybody who puts 35-37 inch tires on their Jeep and claims it didn't change anything is kidding themselves. It absolutely changes everything. Acceleration slows down, braking distance increases, and fuel economy takes a hit. That's just physics. You're asking the engine to turn a longer lever with the same power.

Having said that, I find that after you drive it for a few weeks it'll become second nature to you. The MPG loss will always hurt, but driveability will be something you get very used to. So if you love the look then don't worry about the change in performance.

I would personally advise you NOT to regear a diesel. Diesel engine have very narrow powerbands and peak torque is at 1,500 RPM. The higher the RPM goes, the more fuel economy you will lose. Trust me on this, you want to keep the stock gearing.

 

[CerOf]

For me, I’d only consider re-gearing with 40s.

35s and I don’t really notice any meaningful difference. If I went 37s, I’d notice, but not enough that I’d want to regear.

what I’ve got, diesel jtr on35s is amazing compared to my old JKU rubi on 35s (stock gears).

 

[BEERviper]

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).

 

[stickshifter]

Mopar sells upgraded gears any dealer that says it voids your drivetrain warranty doesn't know what they're talking about:

4.10's factory gears same as a 3.6 Rubicon
4.88 - PN# 77072409 (front)
4.88 - PN# 77072410 (rear)

Good to know!

 

[cecaa850]

Mopar sells upgraded gears any dealer that says it voids your drivetrain warranty doesn't know what they're talking about:

4.10's factory gears same as a 3.6 Rubicon
4.88 - PN# 77072409 (front)
4.88 - PN# 77072410 (rear)

How about if you put an ecodiesel in place of your 3.6? Mopar sells them too. Will your factory warranty still be in effect? If so, how about a hemi?

 

[Msilva]

Those are some nice looking tires though. I will definitely be upgrading when my current tires need replacing. Too cheap to do it before then. LOL

 

[wvuviv30]

How about if you put an ecodiesel in place of your 3.6? Mopar sells them too. Will your factory warranty still be in effect? If so, how about a hemi?

It probably who does the install, are they an Authorized "upgrader"?

When Toyota had the TRD supercharger for the 3.4 and 4.0 Tacomas, if the charger was installed by the dealer, it did not affect the warranty. However, if you order the charger yourself and installed it yourself (or non-approved installer), the engine warranty was voided.

 

[cecaa850]

It probably who does the install, are they an Authorized "upgrader"?

When Toyota had the TRD supercharger for the 3.4 and 4.0 Tacomas, if the charger was installed by the dealer, it did not affect the warranty. However, if you order the charger yourself and installed it yourself (or non-approved installer), the engine warranty was voided.

Mopar accessories are guaranteed for 2 years unlimited mileage I believe. If you install any other random part on your vehicle which was not on it from the factory nor can be added through the Dealer Connect vehicle options area it is considered a modification. Modifications are not covered by warranty regardless who installs the part. Anything damaged due to that part being installed on your vehicle is not covered either. Gearing changes alters emissions which can be another ball of wax.

 

[Daniel Snider]

Well, I ended up going with an all terrain 285-75-17 which I ended up not liking the look of, but did much better I the mpg and power department....

Needless to say, I went back to my 35's and couldn't be happier, I ended up adding the pedal commander and this made ALL the difference in the world. It goes when I hit the pedal and love it.

 

[FutureOdin]

Well, I ended up going with an all terrain 285-75-17 which I ended up not liking the look of, but did much better I the mpg and power department....

Needless to say, I went back to my 35's and couldn't be happier, I ended up adding the pedal commander and this made ALL the difference in the world. It goes when I hit the pedal and love it.

Yup, I got a iDrive and WOW, the difference in the pedal is night and day. Been running mine on auto and it has no problem accelerating when I need it to.

 

[Daniel Snider]

Yup, I got a iDrive and WOW, the difference in the pedal is night and day. Been running mine on auto and it has no problem accelerating when I need it to.

it’s nice to actually be able to buy something and see an instant results to put a smile on your face for not much money.

 

[stickshifter]

Weight is only part of this equation. The additional leverage you've applied to the driveline is the reason it is sluggish now. Anybody who puts 35-37 inch tires on their Jeep and claims it didn't change anything is kidding themselves. It absolutely changes everything. Acceleration slows down, braking distance increases, and fuel economy takes a hit. That's just physics. You're asking the engine to turn a longer lever with the same power.

Having said that, I find that after you drive it for a few weeks it'll become second nature to you. The MPG loss will always hurt, but driveability will be something you get very used to. So if you love the look then don't worry about the change in performance.

I would personally advise you NOT to regear a diesel. Diesel engine have very narrow powerbands and peak torque is at 1,500 RPM. The higher the RPM goes, the more fuel economy you will lose. Trust me on this, you want to keep the stock gearing.

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.

 

[WXman]

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.

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.

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