Help me select my gear ratio

Hi

I currently have a 3.92 differential.

The pinion gear is missing some teeth so I do have to work on the differential.

According to the build sheet I was supposed to have a LSD but it looks like at some point that was pulled out and it is an open differential.

Here’s how I see myself using the truck.

75% commuting.

10% towing a standard 16 foot boat.

15% off-roading. Mostly logging trails, dirt roads etc.

I am going to put in some type of locking differential. Perhaps an Eaton locker.

I am debating a 4” lift. Considering the truck is mainly used for commuting that would be just for looks.

That all said this is the opportunity to change the gear ratio if required.

Should I be sticking with a 3.92 or changing that?

Thanks for the advice.

Alex


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If your not doing a bunch of highway do some 4.56s

How much highway driving, what size tires (diameter is what's important here) and do you have 4x2 or 4x4? 4.56's are great, but yes the highway mileage does suffer. If you have a 4wd you have to change both ends to the same gear ratio so that doubles the cost.

Good questions.

275-60-R20. 33” tires
4X4
Mostly highway. 80% highway 20% City

Thanks for the help.


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What is your engine and trans? I know you have 3.92 gears and the 275/60/20 tires that are that are 33" tall. What engine RPM are you running now at 60mph, 65mph and 70mph?

What is the speed limit along your daily commute?

Do you speed everywhere you go and drive aggressively or are you a conservative driver that runs 5mph under the speed limits?

Having a 4WD makes things more expensive with having to swap the front and rear diffs. Do you have the budget to change the gears in the front axle?

If your budget is limited then stick with the 3.92's and repair the rear diff.

If your budget allows for a front and rear gear change then I suggest the 4.56 gears and drive a more conservative on the interstates.

The 4.56's will get the truck rolling much easier and with less power. The cruise RPM will go up so you will have to drive to match the gears with engine RPM so your not "singin'" down the highway.

The formula to calculate engine RPM at a certain speed is as follows:

Gear Ratio x Speed x 336 divided by tire size x final trans ratio.

4.56 x 70mph x 336 = 107251.2

107251.2 / 33 (tire size) = 3250.03 rpms

3250.03 x .67 (trans ratio in 5th) = 2177.52

2177 RPM at 70 mph in 5th gear with a 33" tall tire.

2021 RPM at 65mph

1866 RPM at 60mph

I ran a 4.56 gear in my 2002 Dakota RC 4.7/45RFE for 4 years on the street with a 28½" E/T Street and I loved it. At 70mph I was cruising at 2800rpm.

I have 4.10's in one of my Ram 1500's and 4.56's in the other with no regrets.

Pick your poison and do your research and math calculations and find the sweet spot for your ride. Good luck and happy modifying.

Hi

At both 60 and 70 mph it was running at about 2000 rpm.

Alex


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Since someone changed things in the rear differential, I would check to make sure the same ratio is in the front diff.

I agree with Old Fart. I would verify that both diffs have the same ratio just for peace of mind.

Now, how did you have the same engine RPM at 60mph and at 70mph?

At 60mph you should be in 5th gear which is a 0.67:1 overdriven gear. At 70mph you should still be in 5th gear.

Your engine RPM in 5th at 60mph should be lower than 5th gear at 70mph.

With the 3.92 gears at 70mph with the 275/60/20's your cruise RPM should be 1871rpm.

At 60 it should be right at 1600rpm

Hi

That’s not what I am seeing.

At 60 mph the tachometer was showing 1950 rpm and at 70 mph it was 1980.

Any idea why the discrepancy?

Thanks




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Ok I went for a second test drive to verify my numbers.

RPMs at 60mph we’re between 1700 and 1750.

Closer.


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Okay good. Those RPM numbers are more accurate. If you swapped out the 3.92's for a 4.56 your engine RPM at 60mph is going to be up around 2000rpm. Not terrible considering the benefits you'll see from the lower ratio.

The only downside is having to do the front diff at the same time because of the cost.

Hi

Still working on understanding the benefit / disadvantage of the various gear ratios.

If I go to a 4.56 in comparison to the 3.92 what’s that going to give me against:

Off-roading
Towing
Hwy
Gas mileage

Thanks

Alex

Fast69Mopar said:

Okay good. Those RPM numbers are more accurate. If you swapped out the 3.92's for a 4.56 your engine RPM at 60mph is going to be up around 2000rpm. Not terrible considering the benefits you'll see from the lower ratio.

The only downside is having to do the front diff at the same time because of the cost.

Click to expand...

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acolic said:

Hi

Still working on understanding the benefit / disadvantage of the various gear ratios.

If I go to a 4.56 in comparison to the 3.92 what’s that going to give me against:

Off-roading - more low end grunt/mechanical advantage for climbing over things
Towing - mechanical advantage, makes it easier to get heavy things moving
Hwy - you will tach more; with a 33" tire and 4.56's I run 2400rpm @ 75mph in 5th gear/TC locked
Gas mileage - better around town since the engine doesn't have to work as hard to move the truck (unless you floor it constantly) but highway mileage will suffer

Thanks

Alex

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Click to expand...

See above

Hi

So Dodge picked the 3.92 ratio because it provides better hwy mileage with lower revs?

Just wondering why they went with the 3.92 over the 4.56. I’d think it would be the same manufacturing cost either way.





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Okay, here goes. The 3.92 is a pretty well rounded gear ratio for our trucks. With our transmission having
2 x overdrive gears and the 5th gear ratio of .67:1 it allows for better acceleration from a standing stop and keeps the engine rpm down at highway speeds. With the street trim weight of our trucks things can get heavy really fast. The 4.56 gear will allow the truck to get rolling so much easier compared to the 3.92 with less throttle input. The torque multiplication will be greater in each gear.

Off road wise you can use 4-HI more because of the lower ratio and still be able to climb and crawl when you want without necessarily having to switch to 4-LOW.

Now, with towing, the lower ratio of the 4.56 will put less stress on your complete driveline and allow your combination to get moving so much easier. Alot of people think that switching to a lower gear ratio will cause you to lose fuel economy. In alot of cases that is true but it doesnt have to be. If you can learn to drive it right and drive for fuel mileage you can make a 4.56 work for your combination and still achieve good results. If Dodge didnt think that was the case they wouldnt have used the 4.56 ratio in the Power Wagon with a 32.7" tall tire like when they used the BFG 285/70/17's.

I have a 2005 Ram 1500 5.7 HEMI Quad Cab truck with the 275/55/20's and a 4.56 gear in the rear and its a blast to drive around town and still run up and down the interstates with ease.

My opinion is stick with 3.92. That is the lowest factory gear option for the max tow rating and plenty low for those tires. Saves you having to do the front diff and calibrating speedo etc. The real question is lsd or locker.

Hi

Why would there be a question between a LSD and a locker?

Given I don’t have either wouldn’t I go with a locker?

Doesn’t a locker like an Eaton provide better traction then a LSD.

thanks for the info.

Alex

BWL said:

My opinion is stick with 3.92. That is the lowest factory gear option for the max tow rating and plenty low for those tires. Saves you having to do the front diff and calibrating speedo etc. The real question is lsd or locker.

Click to expand...

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Lsd has better road manners as it only locks up after some wheel spin. Lockers give better traction, but can be a bit grabby when turning at low speeds. Air or electric actuated lockers keep the wheels locked and are only good off road when engaged and act as an open diff when off.

So are my choices getting a scrap LSD or getting a new locker like an Eaton TruTrac?

Obviously one is a lot is cheaper than the other.


Another note says my pinion gear is chipped is it worth getting a scrap pinion gear and ring gear or are those items best not purchased used?




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If you decide to go used then I'd just look for a low mile 3.92 lsd complete diff. If you know how to rebuild and properly set one up you should be able to remove the cover and see the condition and would be the cheapest solution. I wouldn't buy just one used gear. Apparently the mopar gears are ground different and run smoother and quieter than most aftermarket, but I'm not sure of this. If you decide lsd is best moes sells mopar lsd assemblies and can let you know what other parts you may need so you don't have to go trutrac or eaton if you decide they will be too aggressive for your daily driver. Going new cost will be similar for lsd or any decent locker.