Diesel - Will i regret regearing to 4.88?

[Deleted member 47279]

I made a deal to get some yukon 4.88 gears locally. my Sport has 3.73 gears and its not so bad, but since im installing ARB lockers i figured id regear. i just conflicted if i will lose too much mileage since i normally drive at 75. thoughts? 37" patagonias with a 3.5 metalcloak gamechanger. thanks.

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[bakobobby]

If you are installing arbs you plan to do some moderate plus off roading sooo having larger gears can only help. From what I understand folks see better mileage on 37” and 4.88s gears than stock. Allows the trans to get into the upper gears while stock it has to stay down in the lower gears to move those big tires.

 

[Deleted member 47279]

i figured it would help, but since im 95% on the street going 75 MPH im wondering if im shooting myself in the foot. Thanks!

 

[bakobobby]

Ooh just saw you are diesel. Strike the above advice lol. You won’t wanna have the engine revving that high. You are better off sticking with what you got, sorry.

 

[Deleted member 47279]

Ooh just saw you are diesel. Strike the above advice lol. You won’t wanna have the engine revving that high. You are better off sticking with what you got, sorry.

Appreciate it!

 

[Eazye]

I see about the same mileage going from 4:10 to 4:88 but it’s a better driving experience. It shifts nicer and maintains 7th and 8th on the highway. Although you are staying in the higher gears, you are turning a higher RPM which eats into it maintaining the higher gear. You might be comfortable with 4:56 if it’s highway speeds and mileage you want to maintain. It will still be slightly off from the 4:10/33” tire ratio but will probably be pretty close to the 3:73s with 33” tires.

 

[Eazye]

Just saw you are diesel as well, maybe snag someone’s 4:10 takeoffs if they have them….

 

[Deleted member 47279]

Appreciate it!

 

[bakobobby]

I think you could do well by 4.10. I am not sure what the sweet spot is for those engines but I guess its in the mid 1000 rpm range. I would wait for an expert on the diesel gearing someone here must have real world experience regearing one.

 

[Deleted member 47279]

I think you could do well by 4.10. I am not sure what the sweet spot is for those engines but I guess its in the mid 1000 rpm range. I would wait for an expert on the diesel gearing someone here must have real world experience regearing one.

Thanks!

 

[rharr]

37s with 4.10 on a JTD will get just about back to stock rpm/speed levels at least for a the rubicon with the 3.73 ratio and 33" tires.

http://www.grimmjeeper.com/gears.html

you can use this to compare stock setup to modified setup.

The trick is to be as close to stock rpm/speed levels with the taller tires and regear. it's ok to be a little higher on the rpm (+500rpm but no more) then what the stock rpm to mph is.

We assume the manufacturer has tuned everything to give best power delivery and mpg for the broadest use using the stock gears and tire size.

4.10s are a little much (to much rpm) for 35's last time i ran the calc but could work. 37's and 4.10s brought it to what i am use to stock.

What you are trying to do is fine tune the known performance of the motor and match it to a wanted speed. Basically you are hitting the sweet spot in the torque curve of your motor (dyno chart) where the motor is building the best torque at a certain rpm and then regearing in order to take advantage of that power at that max torque rpm. You need to know what speed you want that max rpm torque delivered at so you can take advantage of it for the work you want.

for example if you are towing and want max torque output at 65 mph so you can pull a hill and not lose power you would look at a dyno chart for your motor type see where max torque happens in the rpm range then pick a ratio that lets you pull at 65 mph while hitting that desired rpm.

You need to look at the gear chart the program provides because it will show all the rpm/speed relationship for each gear and you don't want to pick a ratio that is building max torque at a set speed in the 8th gear say 2500rpms at 65 mph, that will give you quick acceleration but won't have very good top speed or low cruising rpms. Usually you want to use 5th or 6th as your working gears where you get max output for the desired speed.

This is all less important because a diesel builds max torque so low in the rpm range that regears aren't as important that why most can get away with running 35's on a stock ratio, the motor just grunts and turns it.

the gassers build max torque high in the rpm range and the stock ratios keep the rpms down because the market doesn't like a loud high rpm motor singing it's head off all the time. SO you regear to get a little more rpm and closer to the peak torque output at a desired speed, usually highway cruising speed of 65mph. for example you were at say 3000 rpm at 65mph with a stock ratio but now you regeared and you are at 4000rpm (closer to the torque peak) at 65mph, the truck will feel faster because it gets to it power output quicker for the same amount of speed.

enjoy my ramblings....

 

[kevman65]

The tallest gearing I've heard of on the JLD's is 4:18, so I wouldn't venture above that.

I agree the best for your situation would be some 4:10 takeoffs.

 

[Deleted member 47279]

Well i talked with the dude and gave him some ammo for reneging on the deal. thanks everyone! Im going to stay at 3.73 unless i find some 4.10s for cheap.

 

[rharr]

4.10 are plentiful here used as oem take offs. I wouldn't hesitate on using.

 

[DocMcStuffins]

I’ve been moderate/extreme wheeling on 37’s with 3.73s. I didn’t even notice the lack of deep gearing, but that may have been from the JTRD 4:1 transfer case. I do notice it on takeoff though.

I was thinking some 4:10 takeoffs might be perfect, but if I am going through the hassle of regearing I may want to go 4:56 for possible 38-40’s. I have read from a JLUD that 4:88 on 40’s was too low and RPMs too high.

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