CRD : Final Drive, High revving cruise RPM's - JeepForum.com

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post #1 of 29 Old 11-12-2013, 11:02 PM Thread Starter
WKCRD757
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CRD : Final Drive, High revving cruise RPM's

So I immediately noticed upon driving my CRD home yesterday that it revs pretty high at cruise speed. Would a taller tire have a positive effect on MPG numbers at cruise speed or would the effects be cancelled out by having to move a larger tire?

Something like a 31-32in tall tire over the stock 29ish size would lower the RPMS, i'm not sure how much of a difference it would make though in the real world.

I guess the proper question is what is the ideal size for efficiency at cruise before MPG starts declining?

What are your thoughts?


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post #2 of 29 Old 11-13-2013, 06:50 AM
caulk04
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I'd definitely go with a larger tire if I had a CRD. Makes no sense to me why they'd put the 3.73's with the diesel.

If you don't want to lift it, just go with whatever will fit in there. Plenty of threads discussing what fits on stock height. Myself, I'm sticking to stock height tires as I will never do any offroading (nothing but mud where I live) and I want my Hemi mpg to stay as good as possible...lol. 16-17 is tolerable, don't want any worse.

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post #3 of 29 Old 11-13-2013, 06:59 AM
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Keep reading the forum there are a couple of other guys that have done larger tires and do see a bit of a MPG gain if I remember correctly (probably of the 4th or 5th page.

And yeah for me it was really hard to get used to seeing the tack so high when cruising the interstate - 2,500 to about 3,100

You might want to read over the FAQ thread thats sticked at the top, it covers a lot of info with tire sizes and tire sizes with lifts:

http://www.jeepforum.com/forum/f67/f...-wk-xk-703598/


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post #4 of 29 Old 11-13-2013, 07:30 AM
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This is a complete shortsightedness from Chrysler / Jeep as the final drive is way too high for this engine (if this was a gas v6 then it would be fine but not a low end torquey diesel). I spin over 2,500 RPM at 70 mph when not under any load. Larger diameter tires with a small lift will help. You can also change out the rear end / final drive as fishinbama has done here (though he has a 2wd - not sure if it's possible in a 4wd). I just don't go over 70 mph and more often just go 65 mph.

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post #5 of 29 Old 11-13-2013, 10:40 AM
tnadanzig
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Correct, the final drive is way to high for the engine. I run a 255/70/17 tire with zero lift or mods to get the final drive more reasonable, before I install my suspension kit and go to a larger tire package. It drops it about 200 rpm's at 70 mph which is my most normal cruise rpm. I have noticed a small increase ~ .5 -1 mpg while cruising and it stayed about the same in city driving if I take it easy. If drive harder in the city it will go down because of the larger mass and rolling resistance.
I have a set of 3.08 axles for my wife's Jeep with QT2 that I plan on installing this winter to run with stockish size tires to really get a mpg vehicle. Hoping to get about 30 mpg with a few extra mods.

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post #6 of 29 Old 11-13-2013, 12:52 PM Thread Starter
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Oh right on guys, this is good stuff! I do plan on a mild lift, probably the OME HD lift so I guess the taller tire size will help. I'm thinking of a 255 75 17. Dan, what combination are you looking at getting when you lift?

Jeff

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post #7 of 29 Old 11-13-2013, 06:36 PM
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Taller tires will net some fuel economy but there are drawbacks such as increased wind resistance underneath the vehicle and increased rolling weight of the tire. Also, wider tires will lower fuel economy from the great frictional area/wind resistance.

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post #8 of 29 Old 11-13-2013, 09:40 PM Thread Starter
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I think 25 75 17's should be a fairly good choice. They are slim yet they should still do well in the overlanding I plan to do with the Jeep. Now to figure out the lowest weighted tire in that size that is still plenty strong.

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post #9 of 29 Old 12-01-2013, 03:23 PM Thread Starter
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As much as I love the look and performance offroad of the BFG MT KM's they probably aren't the best move for highway MPG increases or will it matter that much? They weigh in at 48lbs in 255 75 17's. There are some other choices a few lbs lighter in some A/Ts. Anyone have any idea on the best compromise? I like the BFG MT's as there are quite a few that pop up on CL with the Rubcon wheels that look so good! Never driven on MT's though, have no idea how loud they are.

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post #10 of 29 Old 01-19-2016, 01:41 PM
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BUMP
I wonder about this every time I drive my wife's CRD on the hwy. Any updates from guys that went to a different axle ratio or bigger tire ?

THANKS
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post #11 of 29 Old 01-19-2016, 01:59 PM
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I run a 255/70/17 (31.1") tall tires and the highway RPMs are down a few hundred. I have the AEV EGR module to calibrate the speedometer so it reads correctly as well. I can still get awesome fuel economy with the tires and a lift, and I actually think it's a bit faster off the line, since the torque has more to pull against.

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post #12 of 29 Old 01-19-2016, 07:40 PM
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I still enjoy our jeep with the 3.08 axle ratio. Now has 110k miles and I plan to see how many we can put on it. It really gives it highway legs.

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post #13 of 29 Old 01-19-2016, 08:42 PM
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i didn't notice any difference going to a 265/75/17 other than rpm is lower. the lower rpm is probably offset by heavier tire and higher ride height
we really needed a 6 or 7 spd trans like the benz's got


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post #14 of 29 Old 01-19-2016, 11:19 PM
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They put 3.73 gears in them to up the gross towing weight.
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post #15 of 29 Old 01-20-2016, 09:05 AM
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fishinbama : your happy with the 3.07 and you would recommend the swap ? drivability off the line 1&2 is still strong? are u running stock tire size ? would this work good with 32"s ?

I think a 6th gear like .6 would be ideal but not really gona happen.

I have a 08 QD2 and am planning a 3.07 32" OME lift but looking for some imput before I get the parts.

thanks
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