Dumb question dont get mad at me

You can have up to 38k and be safe, however that does not mean you are legal (if you are licensed for 34).

 

Can't say about the 38k to 40k difference but 46k and heavier have larger heavier duty components to take the heavier load without wearing out as fast as the lighter components would.

 

While you are limited to 34k running down the road - what about when you are at the customer and they want the tandems all the way back? You can easily hit 38-40k then. You want the axle to be able to withstand normal usage.

 

There's a significant difference between 1/2 ton axles and 3/4 ton axles, and that goes beyond suspension and tires. I've never seen full floating axles in a 1/2 ton, nor have I ever seen weight bearing axles in a 3/4 ton.

A better comparison might be in the 3/4 ton vs 1 ton. Heavier rated axles are going to have beefier bearings to handle the weight, brakes might be bigger to stop the weight, as well as the suspension and tire ratings.

 

38 and 40 k axles are standard highway axles, I don’t think anyone actually makes a 34,000 lbs axle. Up here in Canada we run 37,400 lbs all the time on the same axles.
Generally, even heavy haul still only has the same weights on the drive axles (certainly could be more with the right permits) it’s just that they are pulling a lot more weight. While technically a 38 or 40 k axle can handle the weight placed on it, it is still only rated up to a certain gross combination weight for total lbs being pulled.
A 46k or even 58k axle can pull a lot more weight with out burning up. Dragging all that weight makes a lot of heat.

 

Winner. Beefier axles have beefier wheel bearings, axles and ring gear.

For the most part, most of you wouldn't ever have any issues with the 38k rears. But if you have 1.) Big Power 2.) Bigger than 80k loads on a regular basis 3.) Lot's and lots of big hills to pull, you can easily cook the rear diffs. I always recommend that instead of buying old fleet trucks (only fleet trucks even have 38k rears), to instead buy an owner op spec'd truck with the full gauge package. An owner op truck will have things like a PYROMETER (monitors exhaust temp), engine oil temp sensor, transmission temp sensors and two rear axle temp sensors. These things will let you know if you need to back out of it, or even pull over to cool off BEFORE any parts grenade, saving you huge roadside repair/towing bills.

 

In NC... you can have up to 38,000 on your drives or tandems. But your still restricted to 80,000 total

 

34K on close spaced tandems is only the common denominator. That's what's legal in ALL states. Many states allow more. I guess you haven't purchased a RM motor carrier road atlas and educated yourself regarding the legal size / weight limits for the states you operate in?