2021 40” Rubicon Diesel Build

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40”JLURD

40”JLURD

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By the way this table is also 100% accurate.
Stock I was doing 76 MPH in 8th gear at 2,000 RPM

Now I’m doing 69-70 MPH in 8th gear at 2,000 RPM

The highway MPG is actually hovering right around 20 ish now. But street driving absolutely kills it (starting off from a stop with these big meats).

I haven’t don’t a highway only trip yet long enough to truly get an accurate MPG. But combined since the lift, gears and tires I’ve gone through 3 tanks of diesel with combined driving and haven’t averaged better than 16.6 MPG as
Ok that’s the info I was looking for I really appreciate it. I’m getting 21 average right now on light 40s and stock gears. I couldn’t decide between 4.56 and 4.88.
Of course!

All I ask is if you end up getting the 4.56’s please report back here and let us know how the 3.0 works with them and what kind of MPG you’re getting!





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Ok that’s the info I was looking for I really appreciate it. I’m getting 21 average right now on light 40s and stock gears. I couldn’t decide between 4.56 and 4.88.
Just for reference I’m running 40’ Mickey BAJA BOSS - 92 pounds each
&
17” KMC Grenade Crawl 34 pounds each.

126 pound wheel/tire combo
 

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I haven’t don’t a highway only trip yet long enough to truly get an accurate MPG. But combined since the lift, gears and tires I’ve gone through 3 tanks of diesel with combined driving and haven’t averaged better than 16.6 MPG as hand calculated,and I’ve been driving like I was driving miss daisy too breaking in the new R&P for the first 500 miles.
That's PAINFUL when my 3.6 on 37s is getting 14 city (2k miles and most trips are like 2-3 miles...with warming up over winter). I honestly haven't counted mileage as an aspect yet since so many of my miles are super short trips. I'll let you know how I do once I drive more.
 
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That's PAINFUL when my 3.6 on 37s is getting 14 city (2k miles and most trips are like 2-3 miles...with warming up over winter). I honestly haven't counted mileage as an aspect yet since so many of my miles are super short trips. I'll let you know how I do once I drive more.
Exactly. Basically negated the entire purpose of the diesel motor for me. (To get significantly better MPG than a gasser once done)
 

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But my main beef is the MPG TANKED. Which I was not expecting. That’s why I got the diesel in the first place. I wanted to still be able to get 20+ MPG on 40’s...
...and haven’t averaged better than 16.6 MPG as hand calculated,and I’ve been driving like I was driving miss daisy too breaking in the new R&P for the first 500 miles.
I get that you aren't happy with that number. I've owned 3 different vehicles with 40" tires plus. From my experience I think you should change your opinion to be completely happy. :CWL: :LOL:😎
 
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I get that you aren't happy with that number. I've owned 3 different vehicles with 40" tires plus. From my experience I think you should change your opinion to be completely happy. :CWL: :LOL:😎
It’s all about perception right lol

What were you averaging MPG in those?

Make me feel a little better:LOL:
 

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It’s all about perception right lol

What were you averaging MPG in those?
In my 2018... according to my dash I'm right around what you're getting with the 3.6/Auto/5.38 gears.

My other Jeeps were V8 swaps. My last LS3 Jeep I didn't measure. I didn't pass very many fuel stops. :CWL:
 

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So you're thinking that 4.88 with 40s will be adequate with the diesel? Just wondering
I have heavy 37's on diesel with the stock 3.73's and I can burn rubber no problem. I go up hills and it doesn't even down shift from 8th to 7th. 4.88's on his 40's will be PLENTY
 

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im curious what you have that I don't that is killing your mpg so much. I mean I know I have 37's and you have 40's but I have stock gears and I've added lots of weight including heavy ass rims and im averaging (hand calc) 24.5 and I just did a 760 mile trip mostly all hills, and mostly uphill and my hand calc with the 2 tanks it took was 24.9 going 72mph. that's badass and way better than any gasser I've ever seen in a jeep. note: I also had 350 pounds give or take in the backseat/cargo area of metal. not including me and passengers (310 pounds)
 
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I have heavy 37's on diesel with the stock 3.73's and I can burn rubber no problem. I go up hills and it doesn't even down shift from 8th to 7th. 4.88's on his 40's will be PLENTY
4.88 is way too much. If you want to retain any MPG at least.

im curious what you have that I don't that is killing your mpg so much. I mean I know I have 37's and you have 40's but I have stock gears and I've added lots of weight including heavy ass rims and im averaging (hand calc) 24.5 and I just did a 760 mile trip mostly all hills, and mostly uphill and my hand calc with the 2 tanks it took was 24.9 going 72mph. that's badass and way better than any gasser I've ever seen in a jeep. note: I also had 350 pounds give or take in the backseat/cargo area of metal. not including me and passengers (310 pounds)
Stock gears. Right there is the answer. Since you have increased your tire size but not changed the gears it effectively lowers your final drive ratio which is like having a much less aggressive gear than stock.

For example your 37’s probably measure about 35.5” in reality. If we use this handy dandy calculator at Interco Tire Co. we can see what it does to change tires but not gears.

https://www.intercotire.com/tire_size_change_gear_ratio_calculator


77D217B9-6535-4C7B-9D61-841FF1A4670E.jpeg
F9D95733-073A-4BC5-AC1C-5B79BE55E105.jpeg

So your actual ratio is 3.36 and mine is 4.06

If you look at a typical gearing chart you’ll see you are way into the “Fuel Economy “ area and I’m in the “stock” to slightly over stock area.

How is the power with 37’s and stock gears?

00D039A1-D558-4660-A015-6A88DF2AC0ED.jpeg


It looks like 4.11 and 40” tires would equal just slightly more power than what you have now. That would probably be an ideal combo for fuel economy with 40’s on the diesel.... if it’s got the mustard to move on those big tires under geared like that....
 
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Sorry if I missed it but what gears did you go with and are you happy with them. Looking at 60s for my diesel as well. Thanks
4.11 & 40’s would be a combination close to what guys running stock gears and 37’s getting 25 MPG are on... that may be even better than the 4.56’s ... for MPG....

Stock Gears, 37’s and 25 MPG :mad::swear:
 
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I get that you aren't happy with that number. I've owned 3 different vehicles with 40" tires plus. From my experience I think you should change your opinion to be completely happy. :CWL: :LOL:😎
Look at the post from @Howardlv22 , this is why I’m salty. Other guys are getting like 25 MPG on stock gears with 37’s on the diesel.

I sprung for gears and it appears to have bitten me in the ass for MPG.:angry:

I just filled up an hour ago and it was worst MPG yet 15.67
 

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4.88 is way too much. If you want to retain any MPG at least.


Stock gears. Right there is the answer. Since you have increased your tire size but not changed the gears it effectively lowers your final drive ratio which is like having a much less aggressive gear than stock.

For example your 37’s probably measure about 35.5” in reality. If we use this handy dandy calculator at Interco Tire Co. we can see what it does to change tires but not gears.

https://www.intercotire.com/tire_size_change_gear_ratio_calculator


77D217B9-6535-4C7B-9D61-841FF1A4670E.jpeg
F9D95733-073A-4BC5-AC1C-5B79BE55E105.jpeg

So your actual ratio is 3.36 and mine is 4.06

If you look at a typical gearing chart you’ll see you are way into the “Fuel Economy “ area and I’m in the “stock” to slightly over stock area.

How is the power with 37’s and stock gears?

00D039A1-D558-4660-A015-6A88DF2AC0ED.jpeg


It looks like 4.11 and 40” tires would equal just slightly more power than what you have now. That would probably be an ideal combo for fuel economy with 40’s on the diesel.... if it’s got the mustard to move on those big tires under geared like that....
So this doesn’t change the point you made but my milestar Patagonia 37’s measure 36.8 inches they are ALMOST pure to size. I haven’t regeared yet for the reasons above, crawling, my stock 4low on 37’s has tons of torque and power wise, I feel like this thing is a rocket. I’m going Intake and going to get a tune down the road but 25mpg average is pretty easy with the Ecodiesel and 37’s. I wonder if you would have went 38’s so an inch bigger than my jeep(not 40’s but still great size) and kept your stock gears if you would have been a lot happier
 
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So this doesn’t change the point you made but my milestar Patagonia 37’s measure 36.8 inches they are ALMOST pure to size. I haven’t regeared yet for the reasons above, crawling, my stock 4low on 37’s has tons of torque and power wise, I feel like this thing is a rocket. I’m going Intake and going to get a tune down the road but 25mpg average is pretty easy with the Ecodiesel and 37’s. I wonder if you would have went 38’s so an inch bigger than my jeep(not 40’s but still great size) and kept your stock gears if you would have been a lot happier
Probably. My 40’s measure 38.5” so just under 2” bigger than yours.

I hindsight I do wish I would’ve kept the stock axles and went 38’s.... would’ve been better for so many reasons
 

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