Is it worth regearing my tow rig?

[SnowSTi]

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

[blt2ski]

Not to be a buster on this idea.......Depending upon the gear ring size, going lower than a 4.10 to 4.56 or even a 4.88, may put the teeth too small. So yes, you get use of OD towing. BUT, if you stomp on the go pedal in 1st gear, you may break the R&P teeth! If you have an 8.5" or smaller setup. I would not go smaller than a 4.10. If you have at least a 9.5, then a 4.56 may be the largest you want to go. I really doubt you have one of the 10.5 or larger ring gear setups with a 1500, those, into the 5.xx would be doable.
Also going lower, you may need to change out the axel shaft itself, as the current setup may not handle lower gear sets too! A new 9.5 or larger RP from a 2500 may be in order. Now you have to change out the 8 lug to a 6......any way, I'm going to quit typing, as some of this may be an issue, or it may not!

Just throwing another monkey wrench or 2 or 3 into the thought pattern here!

Marty

[C_Schomer]

I had the same problem with my first 1500 truck and I also ran 3rd gear in a 4 spd auto trans and it worked fine but I would rather have had 4.10s. I never knew anyone who didn't like 4.10s. or even lower, for towing. I loved them in my 03 CTD and I wish I could have found my 12 CTD with them but the 3.73 are a very good in-between for loaded and empty. You'll see a big improvement in acceleration and the truck will do everything easier.

[delwhjr]

SnowSTi wrote:
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

I upgraded my diff to 4.1 and am very happy with results.

[parker_rowe]

SnowSTi wrote:
Looks like 4.88 would probably be more appropriate for my rig. OD would work well for cruising, 3rd would be a good hill gear.

Ok sounds like this is a no brainer especially since the truck is only 2WD so it's not too pricey.

I'll see about getting it done and will report back with the results.

Awesome. Look forward to hearing how it goes. As long as you are OK with turning close to 3K if you ever want to go 70mph, go for it.

You'll be surprised how easy 70 will be to hit even towing after the change. I was.

It will also take a lot of stress off the transmission.

I only did the rear in my truck, and took out the front driveshaft (just in case someone other than me decides to press the 4wd button) until I can get around to the front.

[SnowSTi]

Looks like 4.88 would probably be more appropriate for my rig. OD would work well for cruising, 3rd would be a good hill gear.

Ok sounds like this is a no brainer especially since the truck is only 2WD so it's not too pricey.

I'll see about getting it done and will report back with the results.

[parker_rowe]

time2roll wrote:
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.

It does in mine. We have different torque curves, so I can't say 100% for the OP, but I am pulling a trailer with similar dimensions. I also don't have a T/H mode in my truck.

[SnowSTi]

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]

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]

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]

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]

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]

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]

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.

[parker_rowe]

If you are happy with how it does everything else, I say yes.

Here is my thread on my regear results:

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29728820.cfm

Here is another recent one, OP ended up going for 4:10's and his results are near the end of the thread:

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/29671552.cfm

If you have any questions, let me know.

[Groover]

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.