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gmc 5.3l 1500 sierra 308's want to change to 378 or better

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I bought a 2010 GMC 1500 sierra 5.3 that was to have 342 gears but ended up with 308. I pull a 6000 lb travel trailer. Need advise on changing gears out to 373 or 342. Anyone done this or have advice as to if I should proceed? What cost am I looking at and how much does gas mileage drops. I do have trailer package and tow with tow in while pulling my trailers (travel trailer/bass boat) not at the same time.



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3.73+ or higher, I would stick with factory ratios as you will need to have the dealer re-flash the ECM with the correct factory calibration if you are under warranty, otherwise if you are out of warranty you will need someone to re-calibrate your ECM to correct for the new gears.

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Tranny should definitely run cooler. The higher RPM's help keep it cooler. But beyond 3k rpm's are diminishing returns. I like to stay between 2200-2700 rpm's while towing (cruising). But that's just me.


And the factory installed axillary cooler is not the greatest. If your worried about temps. install a bigger trans cooler. I have a gmt800 but go into my gallery and check out my trans cooler install. Looks factory I think

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I have 3.42s with the 5.3L / 6 speed and I have towed a bumper pull travel trailer in the same weight class. I think the truck did just fine, even in the hills. With the super low 1st gear on the 6 speed you will be miserable with anything lower than 3.73s. If I were in your position it would be a toss up between 3.42 or 3.73


I have also had a 4.8L w/ factory 3.42s and a 4 speed, it was insufficient unloaded and horrible when towing. I swapped it to 3.73s and actually lost no MPG on the highway and picked up 1 mpg in the city. The 3.73s on a 4 speed were a good compromise for the little towing I did but improved driveability without killing mileage.


As for costs....

this can vary greatly. You need a shop experienced in swapping gears, this is not a do-it-yourself type of job (I do motor and transmission swaps, cam swaps, etc. but I don't do gears).

$275 gears

$100 install kit

$ 20 gear oil


Depending on what differential you have you may want to change that too. If you don't have a G80 locker you might as well change that too, that's at least another $400. On top of this you have labor, which will be another couple hundred bucks. I am also leaving out axle bearings and seals, which you can sometimes get away with not replacing, sometimes you can't and that can add up do another couple hundred.


As for coolers:

a little under 200* is normal. Even with a "massive cooler" it won't get much lower, which trying for cooler than 195* would be pointless, this is the temp the transmission was designed to operate at.

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