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Discussion Starter · #1 ·
I've got a 2018 2500 with 35s I'm wanting to change the gears not sure what I need to get close to the mpgs that I was getting before I put the bigger tires on if yall could share what yall have with the 68rfe and 35s rpms and mpg like at 60 70 and 80 mph that would be a big help. I do pull some not sure the weight but is a 20ft flat nosed enclosed trailer. Probably 8 or 9000lbs. Thanks
 

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Not sure on the MPG but in my opinion going from 3.42s to 3.73s would be a waste of money
 
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I am running approx a 33.7” tire now (285/70/18), and after having to replace a transmission recently, I have seriously considered going to different gears, but after much thought, at least for now, I am staying with my 3.73s.
Here is why.
Certainly I suffer a bit on takeoff from stops or at slow speeds, and often I wish I had a 4.30 or 4.56 setup, as I have to be patient and not overload my trans before TC lockup.
But,
At higher speeds when I come to a grade, I don’t wnt to be in my overdrive gears (5 or 6), and 4th gear is the single strongest gear as it is direct drive.
I can hold speeds up to about 63-65mph without the engine revving to high (less than 2200rpm at most effective uphill speeds).

Were I to go to a lower diff gear, my takeoff would be under less strain on the clutches, and overall is easier on the trans.
But,
At hwy speeds (55-70j I would be compelled to run in overdrive gears, or be forced to drop to 4th to get out of the overdrive clutches which are notoriousl weak v. the simple smart decision to attack high load situations (grades) in 4th.
Were I to run 4.56 with my tire size, I would be compelled to climb the same grade 10mph lower than with the 3.73s.
All in all, a give and take, I am pretty happy giving slower takeoff and having to be smart with throttle on low line pressure/ no tc lockup situations (traffic/ stop and go city) and enjoy the benefits of having good speeds in a trusty gear on grades.
The engine has all the power would want. The transmission being the weak link.

I am running a Revmax550 with a revmax stage5 lo-stall TC, and a billet input shaft over Revmax’s TCM tune.
Engine is EFI live Anarchy tune on tow setting.
I tow. 16k 5er, and a 10k car hauler/race trailer.
 
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Not sure on the MPG but in my opinion going from 3.42s to 3.73s would be a waste of money
As for this, I would consider the speeds you run, and the rpms you are comfortable running.
Further, I would keep in mind the good prudence of staying out of the overfpdrive gears under heavy load and see where 4th gear puts you re: speed v. rpm
 
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Discussion Starter · #5 ·
I'm looking at how the truck ran before I put the 35s on I went from 21 or 22 mpg hwy to 15 or 16 hwy. From the ratio Calculator I used going to a 3.73 gear would compensate for the 35s and be a little bet lower than stock. I do take this truck on alot of long trips and am afraid that 4.10s would be to low for 80 mph but don't know. I know there is no magical gear that would do every thing I want but just trying to get some numbers from you guys to see what's best. I would like to be able to get back closer to 20 mpg hwy and tow a little better than stock.
 

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Changing gears does more than just relieve strain on take off. It reduces load across all speeds and rpms.

To the OP, gears will have a minimal affect on mpgs at 80mph. Anything over 70ish and they mileage on these trucks tank due to the wind drag.

35s are not much larger than stock. 3.42s are pretty terrible for anything other than unloaded highway cruising mileage.

Have you been hand calculating the mileage or using the evic? The evic is not known for being accurate.. and if you drive north or 70mph regularly 15-16 may not be too far off.
 

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I'm in the exact same boat with my '13. I have a carli 3" with 35s and want to go from the stock 3.42s to 3.73s like are in my '11.
 

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I still get like 20mpg on the freeway...but I drive like an old fart....pretty much 65.

If you drive 75 or 80 your mpgs will suck.

Overall, I love em and glad I got them...I dont tow a ton, but it tows like a champ (light boat or 5th wheel)

Sent from my SM-N950U using Tapatalk
 

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You can use this calc to input tire size, final trans ratio, and diff ratio along with mph and will yield your rpm

Use the bottom table

Engine RPM Calculator
 

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I've used a few Calculators but it's about 100rpms off for my truck so I was just trying to get some real world numb
This is “real world”

You need to know your transmission final drive

6th in a 68RFE is 0.63

4th is 1.00

If you have TC lockup, your tire height measurement is accurate, and you plug in the numbers correctly this is far more precise than looking at a tachometer that is divided into 200rpm increments.
 
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