Determining Rear GAWR

[cbwallace]

I was reading another post and it got me wondering... I have a 2013 2500 HD with a rear GAWR of 6200 lbs. I have 265/70R18E Michelin's with a max load of 3525@80 psi per tire giving me a tire load capacity of 7050 per axle. The 3500's use the same axle and came standard with the 265/70R18E Michelin's, but the 3500's have a rear GAWR of 7050. Is there a difference in the load capacity of the wheels? If not, why the difference in rear GAWR? I also have to wonder why GAWR doesn't differ depending on what tire option a truck has from the factory.

In theory, shouldn't I be able to safely handle a rear GAWR of 7050 if I had air bags?

For the record, I have been to the scales and I am well within my weight ratings!

[cbwallace]

BenK wrote:
You should rename the title to: "how can I change my rear GAWR"

Didn't mention the third 14 bolt in the GM parts bin, because of your
TV's era. Of the three 14 bolts in the GM parts bin that I know of,
are the 9.5" ring dia, 10.5" ring dia, and the 11.5" ring dia

Within all of them are varying parts bolted on by GM from the base
axle assembly supplied by an external OEM

GM rates these from 6K GAWR to 10K GAWR....IIRC

Folks who can eyeball a leaf spring to know it's static pre-stressed arch;
thickness; tapper; width; etc have better eyeballs than I do...

Yes you *CAN* change the GAWR, but it might cost you more than going
out and buying a new truck

The ME/PE (mechanical engineer, profession engineer) will need to
redesign and then certify their engineering. Then a shop will need to
make whatever mod's per that new specification. Then have all that
checked by the ME/PE and whatever certification agency deemed necessary

Then they will make up a new door label for you with the new GAWR

As for the PSI loading on the pavement, yes very true and accurate,
but that is only one in the food chain from the frame down to pavement

Changing one component of the system 'might' do it, but then you will
need to know that it was the weak link in that food chain down to pavement...

To then find the 'NEW' weak link that will then become the limiting
factor of that food chain...and so no and so on...

I understand overloading a rear axle beyond it's RGAWR...I replace
the rear axle bearings on my 1980 Silverado often, as I over load it
often. Do have 1 ton helper coil springs on the axle bump stop perch
to frame. It does not squat much with a +2,400 lb load. Do have
performance level braking and 33/12.5R15LT load range C tires on
15x8.5 alloys.

That kind of overloading doesn't break it instantly, but takes the
axle bearing out in a couple of years...dependent on how often...how
much over...how fast I've driven over loaded...etc

cbwallace wrote:
I was reading another post and it got me wondering... I have a 2013 2500 HD with a rear GAWR of 6200 lbs. I have 265/70R18E Michelin's with a max load of 3525@80 psi per tire giving me a tire load capacity of 7050 per axle. The 3500's use the same axle and came standard with the 265/70R18E Michelin's, but the 3500's have a rear GAWR of 7050. Is there a difference in the load capacity of the wheels? If not, why the difference in rear GAWR? I also have to wonder why GAWR doesn't differ depending on what tire option a truck has from the factory.

In theory, shouldn't I be able to safely handle a rear GAWR of 7050 if I had air bags?

For the record, I have been to the scales and I am well within my weight ratings!

I am not actually looking to change my GAWR just trying to understand what components are different in the calculation that lead to some of the disagreeing points of view. 🙂

[wintersun]

The 2500 truck will have leaf springs that support less weight than those on the 3500 SRW truck which will have less than that of the DRW version of the 3500 truck.

The factory calculates the GAWR based on how the truck is configured when it leaves the factory and based upon the weakest component. For most trucks it is the leaf springs followed by the tires and rims. Both can be upgraded for the purpose of carrying a greater payload.

Easiest way to retain the handling of the truck and increase the payload is with Supersprings that take an hour to install and will outlast the truck. They work just like the factory overload leaf pack that is installed at the factory on the 3500 trucks.

Air bags are good when you need to be able to adjust the rear height of the bed for towing a 5th wheel trailer.

[BenK]

You should rename the title to: "how can I change my rear GAWR"

Didn't mention the third 14 bolt in the GM parts bin, because of your
TV's era. Of the three 14 bolts in the GM parts bin that I know of,
are the 9.5" ring dia, 10.5" ring dia, and the 11.5" ring dia

Within all of them are varying parts bolted on by GM from the base
axle assembly supplied by an external OEM

GM rates these from 6K GAWR to 10K GAWR....IIRC

Folks who can eyeball a leaf spring to know it's static pre-stressed arch;
thickness; tapper; width; etc have better eyeballs than I do...

Yes you *CAN* change the GAWR, but it might cost you more than going
out and buying a new truck

The ME/PE (mechanical engineer, profession engineer) will need to
redesign and then certify their engineering. Then a shop will need to
make whatever mod's per that new specification. Then have all that
checked by the ME/PE and whatever certification agency deemed necessary

Then they will make up a new door label for you with the new GAWR

As for the PSI loading on the pavement, yes very true and accurate,
but that is only one in the food chain from the frame down to pavement

Changing one component of the system 'might' do it, but then you will
need to know that it was the weak link in that food chain down to pavement...

To then find the 'NEW' weak link that will then become the limiting
factor of that food chain...and so no and so on...

I understand overloading a rear axle beyond it's RGAWR...I replace
the rear axle bearings on my 1980 Silverado often, as I over load it
often. Do have 1 ton helper coil springs on the axle bump stop perch
to frame. It does not squat much with a +2,400 lb load. Do have
performance level braking and 33/12.5R15LT load range C tires on
15x8.5 alloys.

That kind of overloading doesn't break it instantly, but takes the
axle bearing out in a couple of years...dependent on how often...how
much over...how fast I've driven over loaded...etc

cbwallace wrote:
I was reading another post and it got me wondering... I have a 2013 2500 HD with a rear GAWR of 6200 lbs. I have 265/70R18E Michelin's with a max load of 3525@80 psi per tire giving me a tire load capacity of 7050 per axle. The 3500's use the same axle and came standard with the 265/70R18E Michelin's, but the 3500's have a rear GAWR of 7050. Is there a difference in the load capacity of the wheels? If not, why the difference in rear GAWR? I also have to wonder why GAWR doesn't differ depending on what tire option a truck has from the factory.

In theory, shouldn't I be able to safely handle a rear GAWR of 7050 if I had air bags?

For the record, I have been to the scales and I am well within my weight ratings!

[blt2ski]

Legal you get 500 lbs per inch width of tire to 20K lbs per axel, ASSUMING, you have enough paid for license registration.....so a 265 width tire is about 10.5" or 5250 per tire, for a total of 10,500 per axel......

Warranty and performance factors, ie engineers ratings or what ever is a whole nuther ball of wax.

The end is what the spec needs to be, how long something is to last etc. The tire OP has, is good to 3700 lbs at speeds up to around 70 mph. If he is to stay below 25mph, you would find the tire manufacture would rate the tire at about 4500-5000 lbs. Doing nothing else to the tire.

Reality is, in this case as noted, the engineer difference is the spring pack!

Marty

[Thunderbolt]

I believe the 3500 has one more leaf spring than the 2500HD.

cbwallace wrote:
I was reading another post and it got me wondering... I have a 2013 2500 HD with a rear GAWR of 6200 lbs. I have 265/70R18E Michelin's with a max load of 3525@80 psi per tire giving me a tire load capacity of 7050 per axle. The 3500's use the same axle and came standard with the 265/70R18E Michelin's, but the 3500's have a rear GAWR of 7050. Is there a difference in the load capacity of the wheels? If not, why the difference in rear GAWR? I also have to wonder why GAWR doesn't differ depending on what tire option a truck has from the factory.

In theory, shouldn't I be able to safely handle a rear GAWR of 7050 if I had air bags?

For the record, I have been to the scales and I am well within my weight ratings!

[Me_Again]

cbwallace wrote:
I was reading another post and it got me wondering... I have a 2013 2500 HD with a rear GAWR of 6200 lbs. I have 265/70R18E Michelin's with a max load of 3525@80 psi per tire giving me a tire load capacity of 7050 per axle. The 3500's use the same axle and came standard with the 265/70R18E Michelin's, but the 3500's have a rear GAWR of 7050. Is there a difference in the load capacity of the wheels? If not, why the difference in rear GAWR? I also have to wonder why GAWR doesn't differ depending on what tire option a truck has from the factory.

In theory, shouldn't I be able to safely handle a rear GAWR of 7050 if I had air bags?

For the record, I have been to the scales and I am well within my weight ratings!

The RGAWR is most likely set at the lower number based on the base tires available on that model and/or the lighter spring pack!

You already do not have the base tires, so helping the springs to keep the loading level will result in similar results vs the 3500 SRW.

Chris

[ib516]

ScottG wrote:

ib516 wrote:
My buddy has a 2014 Ram 3500 SRW/Cummins 4x4.

It has the exact same wheels and tires (size, type, brand) as my truck, but has 500# more rear GAWR (6500# to his 7000#). Must be the springs.

If your friend has an 2014 HO the ring gear in the diff is bigger than yours and either way, he has different and bigger wheels (18") than you and while I'm not positive, I think he has bigger brakes than your 07 has.

I have a 2014 now....exact same wheels and tires (18" Firestone Transforce HT) on my 2500 gasser and his 3500 SRW Cummins. The brakes appear to be the same by visual inspection, though I haven't compared part numbers to be sure. The tires have a max load of 3640# each @ 80 psi. 3640# x 2 = 7280#, but my rear GAWR is 6500#, his is 7000#, both are under max load for the tires.

His is not the HO, it is a 370/800 68RFE 3.42.

[fla-gypsy]

BenK wrote:
another factor is the diff ring gear dia and the subsequent axle
bearing size. There are TWO 14 bolt diff's that GM puts in +8K GVWR trucks/SUVs

Note that the OEM that supplies the rear axle assembly for my GMT400
rates it at 10,000 lb GAWR

so why does GM rate it for 6,000 lb GAWR?...yes, that is the sum of
the tire's max weight rating, but does that mean if I put in 4,000 lb
rated tires it would now be rated 8,000 lb GAWR?

Mine is also the same axle assembly for the 1 ton dually of my year and it
too is rated 6,000 lb GAWR...so the exact same?

Nope, the MC bores are larger on the 1 ton dually. The rear brake drum
dia are the same, but some 8,600 GVWR Sub's have that same dia, but
2.5" wide shoes/drums...while others and the 1 ton dually has the
same dia drum, but 3.5" wide shoes/drums. The rear brake cylinder bores
are also larger. The brake proportioning valve body are also different

It's not any one component of the 'system', but the whole system that
dictates

Benk illustrates well why none of us can make assumptions about why the GVWR and axle ratings are what they are. If you just stick to what the manufacturer says is the maximum rating for each of those things you do not have to be overly concerned.

[JIMNLIN]

I also have to wonder why GAWR doesn't differ depending on what tire option a truck has from the factory.

The difference is tires/wheels and rear spring pack. Generally on the GM and Dodge/Ram it can also be the 3500 SRW has a aux overload spring pack.
You have the optional 18" tires and wheels as the 3500 SRW uses. If your truck doesn't have the aux overload spring pack then it doesn't have the same capability as the 3500 SRW truck.

Adding bags does give your truck more payload ability but just guessing if air bags would give a 2500 rear suspension a same as.

[ScottG]

ib516 wrote:
My buddy has a 2014 Ram 3500 SRW/Cummins 4x4.

It has the exact same wheels and tires (size, type, brand) as my truck, but has 500# more rear GAWR (6500# to his 7000#). Must be the springs.

If your friend has an 2014 HO the ring gear in the diff is bigger than yours and either way, he has different and bigger wheels (18") than you and while I'm not positive, I think he has bigger brakes than your 07 has.

[ib516]

My buddy has a 2014 Ram 3500 SRW/Cummins 4x4.

It has the exact same wheels and tires (size, type, brand) as my truck, but has 500# more rear GAWR (6500# to his 7000#). Must be the springs.

[BenK]

another factor is the diff ring gear dia and the subsequent axle
bearing size. There are TWO 14 bolt diff's that GM puts in +8K GVWR trucks/SUVs

Note that the OEM that supplies the rear axle assembly for my GMT400
rates it at 10,000 lb GAWR

so why does GM rate it for 6,000 lb GAWR?...yes, that is the sum of
the tire's max weight rating, but does that mean if I put in 4,000 lb
rated tires it would now be rated 8,000 lb GAWR?

Mine is also the same axle assembly for the 1 ton dually of my year and it
too is rated 6,000 lb GAWR...so the exact same?

Nope, the MC bores are larger on the 1 ton dually. The rear brake drum
dia are the same, but some 8,600 GVWR Sub's have that same dia, but
2.5" wide shoes/drums...while others and the 1 ton dually has the
same dia drum, but 3.5" wide shoes/drums. The rear brake cylinder bores
are also larger. The brake proportioning valve body are also different

It's not any one component of the 'system', but the whole system that
dictates

[RoyJ]

And this is what I hate regarding engineering vs marketing vs legal specs!

Any engineer would tell you that if you eliminate the weak spot - tires + wheels, then you should be good for the maximum weight the actual axle is rated for.

So physically, I can guarantee you have nothing to worry about. Legally? Who knows...

I will say, as an engineer myself, if I were ever called into court as a "subject matter expert", I would wholeheartedly defend the guy who put 19.5s on a 3500 SRW, and loaded up the truck to DRW weight limits.

If you dig deep enough, I'm sure you can find the engineering documentation at GM stating the 6200 lbs being a tire limit. If you coupled that with the real axle rating from the axle manufacturer, you'll be able to legally defend yourself.

[Dayle1]

gmcsmoke wrote:
there's no difference in the wheels the rear axle is derated to fit under the lowered GVWR. The FAWR is the same for both 25/3500 trucks.

Actually, most pickups have GVWRs that are significantly less than the total of the GAWRs.