Forum Discussion

SnowSTi's avatar
SnowSTi
Explorer
Aug 30, 2018

Is it worth regearing my tow rig?

I recently bought a new travel trailer and a used truck to use as the tow vehicle. When I bought the truck I was told it had a 3.73 rear diff... anyhow after some investigation it has the 3.42. That drops the tow capacity down to 6700lbs from 7700lbs which is cutting it close for my rig ( Here are the details )

2008 GMC Sierra 1500 Crew Cab Shortbed
5.3L / 4L60e / 3.42 diff / 2WD / G80 locker
Trailer package
Heavy Duty rear suspension
Upgraded Transcooler to 24,000 GVWR (Hayden 678)

Jayco 267BHS
5690Lbs dry / 6200Lbs loaded
Equalizer WDH

The truck tows the trailer well in 3rd gear on the highway at 2400 - 2500rpm. Never downshifts unless it's a very steep hill. Acceleration is slow but acceptable. Transmission temps hover between 70C - 80C while towing during the summer.

I've done some smaller 2-3 hour trips already with the rig and it will cruise along quite happily on the highway. However I've got a longer 4000km trip later this year which has me thinking a bit.

Would going to a 4.10 from the 3.42 be worthwhile? It would move my cruise rpm up to 3000 in 3rd which seems unnecessarily high, but I assume would make accelerating a bit better.

It would cost me about $1000 to get the rear diff swapped, plus I would then need to get a programmer to correct the speed/transmission shift points.

The rig in question
Tow Vehicles

30 Replies

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  • SnowSTi's avatar
    SnowSTi
    Explorer
    Aug 30, 2018
    I'm going to have to play around with it the next time I tow to see if 2300rpm is enough. I'm starting to lean towards 4.88. 2500rpm is definitely the sweet spot when I'm cruising.

    Seems like a massive ratio change, but the calculations all look good.

    @ 65mph

    4.88 - 3rd gear - 3500rpm
    4.88 - 4th gear - 2450rpm
  • time2roll's avatar
    time2roll
    Nomad
    Aug 30, 2018
    Is 2300 rpm enough to hold OD? 4.88 puts you at 2461... about same as now.
    Hard to know as I have not driven it. If you are solid at 2300 then the 4.56 is fine.
  • SnowSTi's avatar
    SnowSTi
    Explorer
    Aug 30, 2018
    Groover wrote:

    With a 4 speed transmission you could end up in a situation where with the new rear end you will get 2000rpm in 4th and 2800rpm in 3rd at highway speeds and neither one is really comfortable. You may want to evaluate that before making a decision. "Never downshifts unless it's a very steep hill. Acceleration is slow but acceptable." You may already be in your sweet spot.


    That is exactly my concern.

    I've run a few ratios through and get the following at 65mph.

    3.42 - 3rd gear - 2450rpm
    3.42 - 4th gear - 1700rpm

    4.10 - 3rd gear - 2950rpm
    4.10 - 4th gear - 1900rpm

    4.56 - 3rd gear - 3250rpm
    4.56 - 4th gear - 2300rpm

    Seems like a 4.56 makes more sense as I may be able to use OD.
  • parker_rowe's avatar
    parker_rowe
    Explorer
    Aug 30, 2018
    4.56's and 31.5" tires put me at 2200-2300 @ 65mph. I am very glad I didn't go with 4.10's.

    Here is a great link for calculating speed and rpm I'm different gears:

    Gear Ratio Calculator

    I have the same transmission as you, but less power at lower RPMs.

    I would expect a 5.3 to appreciate a little gear since it makes power at a higher rpm than my 5.7.

    I gained .5-1mpg towing. Haven't checked unloaded, probably lost about the same.

    SnowSTi wrote:
    Yeah I would need to go to a 4.56 to make use of OD I think. Looks like I'd be at roughly 2300rpm at 65mph using 4.56 gears. That probably makes more sense than the 4.10s.

    This truck is used exclusively as a tow vehicle so I don't care how it performs when it isn't towing.


    I would do 4.56's in a minute then. Best bang for the buck change I've made to my truck so far.
  • SnowSTi's avatar
    SnowSTi
    Explorer
    Aug 30, 2018
    Yeah I would need to go to a 4.56 to make use of OD I think. Looks like I'd be at roughly 2300rpm at 65mph using 4.56 gears. That probably makes more sense than the 4.10s.

    This truck is used exclusively as a tow vehicle so I don't care how it performs when it isn't towing.
  • time2roll's avatar
    time2roll
    Nomad
    Aug 30, 2018
    SnowSTi wrote:
    The truck tows the trailer well in 3rd gear on the highway at 2400 - 2500rpm. Never downshifts unless it's a very steep hill.

    Would going to a 4.10 from the 3.42 be worthwhile? It would move my cruise rpm up to 3000 in 3rd which seems unnecessarily high, but I assume would make accelerating a bit better.

    I would aim for 2500 - 2700 in OD. Not sure of your transmission ratios but that would probably put you in 4.56 or 4.88 ratio.
  • dodge_guy's avatar
    dodge_guy
    Explorer II
    Aug 30, 2018
    If you have 3.42 then going to 4.10 will allow you to tow in OD. Best thing I’ve done. I’ve even heard of people going to 4.20 4.30 ratios. I went from 3.73 to 4.30. Best thing I have done. It let me use OD when towing.
  • Groover's avatar
    Groover
    Explorer II
    Aug 30, 2018
    You are looking at a 20% difference in the ratios. Changing ratios will help you in first and reverse gears then just trigger earlier shifts to the other gears. You would be coming close to trading overdrive for better low end power. It would help when pulling but then you would be running about 2300rpm on the highway empty. It comes down to how often you tow, would you be happy with that engine speed empty and how important is $1,000 to you. It sounds like you bought the truck just for towing the trailer so empty running may not even be a factor.

    On the level highway it is more about frontal area than weight so I wouldn't get too wound up over that and you have improved transmission cooling. Just watch for situations that cause your transmission to slip and keep them to a minimum. When climbing steep hills I will often force a downshift to take some of the torque off of the transmission.

    With a 4 speed transmission you could end up in a situation where with the new rear end you will get 2000rpm in 4th and 2800rpm in 3rd at highway speeds and neither one is really comfortable. You may want to evaluate that before making a decision. "Never downshifts unless it's a very steep hill. Acceleration is slow but acceptable." You may already be in your sweet spot.
  • anw7405's avatar
    anw7405
    Explorer
    Aug 30, 2018
    If everything works as is I wouldn't touch it. There is nothing wrong with towing in 3rd and there is nothing wrong with 70-80C trans temps. Now if you go with a bigger trailer that would be a different story.

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