Anyone changed their rear end for more towing capacity?

westernrvparkowner wrote:
" > Anyone changed their rear end for more towing capacity?" My mistake, I thought this was a thread about a new diet plan.

HARR harr har - me too :)

Rob,

You know I agree with you, right?

I am actually expecting a slight decrease in fuel economy not an increase.

As for the increased capability of the truck after the installation of a new gear set - I know that there is no linear relationship with rating, and said so. I merely was bringing up the rating of a factory 3.42 truck for comparative purposes for what can expect regarding tow ability (and power train longevity, perhaps) before vs after the mod. Of course changing to a lower gear as the need arises will still be there, rear end gearing notwithstanding.

Ratings are not simply due to gearing capability, but must also consider the entire capability of the power train as a whole over the period the manufacturer is prepared to warrant and (I would expect) the reasonable life of he truck to the owner, along with other less tangible aspects (again, I would expect) as towing ease and confidence, etc. lets not forget marketing influences as well.

BTW, all of this is my opinion as a practicing engineer since like most here, I have no direct insight into how manufacturers assign ratings.

wmoses wrote:

Even if the rating were linear, it would mean that 7000# becomes 7700# which is greater than the GVWR of the RV I am contemplating, while at the same time not impacting my fuel economy as much as a lower gearing would. Since this truck is my daily driver that is very important.

I wouldn't get too hung up on fuel economy differences between these two rear ends. Really, I wouldn't. The change is very minor (11%). All it means is with one you'll be spinning 11% less RPMs. That does not equate to 11% better fuel economy. In fact, it may equate to worse fuel economy, depending on the type of driving you do. If you do a lot of stopping and starting, as I do, a lower (numerically higher) gear is more advantageous to fuel economy. If you drive all freeway, the opposite is true. That said, I'd be very surprised if either of those two rear ends produced any measureable difference in fuel economy.

wmoses wrote:

The reality is, if GM can rate their 3.42 geared truck with my configuration at 9600# (+37%) then the relationship is not linear with respect to rating.

I don't think the relationship of "tow rating" is linear with respect to anything since it doesn't make a lot of sense to begin with. There's no magic going on here. The laws of physics say you'll get 11% more torque, the "tow rating" number says you can tow 37% more weight. Which do you believe?

wmoses wrote:

At the end of the day, I would be going for more than the RV GVWR that the new ratio can give - whatever that margin is - and I still hope to get close to the usual non-towing 17 mpg average I get now.

Try dropping a gear while towing. Like I said, it'll give you a lot more torque, and it's free.

-- Rob

bimbert84 wrote:
I think you should evaluate your expectations in changing from a 3.08 to a 3.42.

3.42 / 3.08 ~= 1.11

Tow ratings aside, this means that at any given speed, the 3.42 will put 11% more torque to the wheels and turn 11% higher RPM.

11% is not much of a change IMO. Unless you're having trouble launching from a stop, I'd keep the 3.08 and try downshifting a gear in the transmission. That'll give you a much bigger increase in wheel torque, and best-of-all, it's free.

-- Rob

Rob,

Thanks for the input.

Even if the rating were linear, it would mean that 7000# becomes 7700# which is greater than the GVWR of the RV I am contemplating, while at the same time not impacting my fuel economy as much as a lower gearing would. Since this truck is my daily driver that is very important.

The reality is, if GM can rate their 3.42 geared truck with my configuration at 9600# (+37%) then the relationship is not linear with respect to rating. At the end of the day, I would be going for more than the RV GVWR that the new ratio can give - whatever that margin is - and I still hope to get close to the usual non-towing 17 mpg average I get now.

I think you should evaluate your expectations in changing from a 3.08 to a 3.42.

3.42 / 3.08 ~= 1.11

Tow ratings aside, this means that at any given speed, the 3.42 will put 11% more torque to the wheels and turn 11% higher RPM.

11% is not much of a change IMO. Unless you're having trouble launching from a stop, I'd keep the 3.08 and try downshifting a gear in the transmission. That'll give you a much bigger increase in wheel torque, and best-of-all, it's free.

-- Rob

wmoses wrote:

blt2ski wrote:
The rockies will test you because for one, depending upon the elevation you are at when there, lets say 10K' like vail pass or the highest point in Yellowstone, with a std gas v8, you will be down 2-3% HP per 1000', so 20-30% on you hp/torque in the rockies! There is NOT a gear ratio change to make up this difference. THe only TRUE real way to fix this is a super or turbo charger on the motor! THen you can get to 10K' or so with out losing any HP/torque from the motor.

Very true.

So now I am toying with this reasoning .... if I am willing to spend $1000 to change out the rear end, why not consider taking the truck to Hennessey Performance here in Houston and let them put in a supercharger! :D They happen to do drivetrain upgrades as well, and offer a 3/36000 warranty. Consider the smile on my face when not towing ... :D Might even be less money than a new truck.

Reality is, IF the SC can be added, it will do more from a performance standpoint than gears in the pumkins will do! Reality is, if pulled over from an LEO, they do not know, or care what your tow rating is. ALong with they have no way to find out. THey do not even know your gcwr. If you are in my state, they do not care what the door sticker gvwr and gawr's are! I get 500-600 lbs per inch width of tire, as long as my paid for gvw is greater than I weigh, and I have less wt per tire width inch than 500 lbs. You would have to be close to 20K lbs or 10K per axel to be over wt on your rig!

MB in the ML 320 suv my wife has, the tow rating is 5K lbs. We have about a 200/200 motor. I can get an ML55 model, which has a 5.5L V8 with a super charger, putting out in the area of 400/400, and its tow rating is ALSO 5000 lbs! Fine print, get the higher hp/torque motors for better speed performance, for up hills acceleration etc! I can find other manufactures here and there that use the same formula per say for tow ratings. BUT, most seem to use a useless engine/axel ratio BS for tow ratings/gcwr! Which from experience, I have had a truck with a gcwr LESS than its gvwr, go up a steeper grade at 150% of the ratings, meanwhile a BB V8 with over double the HP and 30% more torque, could not go up the same hill at the same wt and 25% LESS than its gcwr. So, do you think I trust tow or gcwr ratings!!!! NOT on you life! pure bonus bogus BS IMHO!

At the end of the day, one needs a tow rig that has the payload for people, other gear, hitch wt etc, proper power and gears in combo with axl ratio and trans, trans IMHO is more important than the axel ratio as shown by the excel program APT has posted! THEN and maybe THEM, you should have a rig that will do what you want it to do. Until them, you may be VERY disapointed in teh R3500 dually with a BB V8, TH400 tranny, 4.10 gears etc, as the worst poj you have ever owned! It could not even out pull a 2500 with a 105HP 292 I6 motor!

So if it were me, I would put the money into a super charger if you are going into the mtns, ie above 5-6K feet frequently. If not, but maybe once in the time you own said truck. Deal with the slower speeds you will be doing in the rockies, and enjoy the view as you go slower!

Marty

Yup...me too noodling forced fed big block (in my dreams for now)

Never going to get a newer TV because of the over loaded bells and whistles....ever higher complexity, etc. Plus love what I
have...along with not having payments since before the turn of the
century

Ops...did with the Benz S class and Lexus L class...but paid them off and turned
them over to the 'ex' when the paper work making her an 'ex' was finalized...

Am for sure going with a drive shaft OD unit (even though it's a 4x4...just chose
wisely so that it can take the torque in compound low)

Since have 4.1's and a big block...force feeding it a natural for boy racer stuff
out on the boulevard...issue will be with several tons on it's tail going up
inclines from sea level to +10K feet

Inter cooler (really big guy) and maybe an internal engine oil spray system
would do the trick

Now if only can get a hold of the prototype solid state AC compressor we invented
for computer chip cooling... :)

blt2ski wrote:
The rockies will test you because for one, depending upon the elevation you are at when there, lets say 10K' like vail pass or the highest point in Yellowstone, with a std gas v8, you will be down 2-3% HP per 1000', so 20-30% on you hp/torque in the rockies! There is NOT a gear ratio change to make up this difference. THe only TRUE real way to fix this is a super or turbo charger on the motor! THen you can get to 10K' or so with out losing any HP/torque from the motor.

Very true.

So now I am toying with this reasoning .... if I am willing to spend $1000 to change out the rear end, why not consider taking the truck to Hennessey Performance here in Houston and let them put in a supercharger! :D They happen to do drivetrain upgrades as well, and offer a 3/36000 warranty. Consider the smile on my face when not towing ... :D Might even be less money than a new truck.

APT wrote:
Wmoses,

Isn't your TT like 5500 pounds dry? You are likely in the 6500 pound loaded range for a couple's worth of gear.

The TT I am looking at is 6100# dry with a 7550# GCWR.

My trailer payload estimate is based on what I carry in my present RV which is about 3000# dry and 3800# as weighed. That includes some mods, propane, and everything else for a functioning RV.

APT wrote:

gmcsmoke wrote:
your spreadsheet is wrong the 6L80E has a 1.2 4th gear

Where did you find that? I have seen several references to 1.15.


Wmoses,

Isn't your TT like 5500 pounds dry? You are likely in the 6500 pound loaded range for a couple's worth of gear.

the fleet ordering guide and GM powertrain have it listed as 1.2