lifter axle and legal weights

It would depend on the specs of the axles on the truck. They could be spec'd for less than the 34K limit, if so you would be limited to the spec, or possibly higher than 34K which then you would be limited to the local regs. Is it a full size class 8 tractor, or a smaller class truck stretched with an extra axle?

 

Hope I'm understanding this quest right...

IF it's on tires and axles common to big-rigs. (not a p/u)

You are allowed 12,000 on the steer, & 20,000 on a single drive axle for a total of 32,000 lbs on the tractor. Single axle trailers are 20,000 lbs per axle.
On a"tandem" (set spaced less than 10'2" apart) your allowed 34,000 lbs on that set. Add that to the 12,000 for the steer you now have 46,000 on the tractor. Add the trailers set at 34,000 and now you have a total of 80k.
Doesn't matter if it's a "tag-axle" or not as long as it's part of a 2 axle set spaced less than 10'2".

 

If the axle is rated accordingly, as well as the tires, then it will be a normal weight axle. So 17,000 per axle assuming a 5 axle setup. 80k lbs gross.
If the axle is only rated at 12k lbs though, then it will drop.
Should be a sticker on the drivers door jamb that will list the axle weights and the gross weight of the tractor only. Normally this is 52k lbs with 40k lbs rears.
To be safe, look at the sticker from the manufacturer.
I have thought long and hard about a lift instead of the twin screw, but that is where it stops, in my thoughts....

Martin

 

That is why most trucks with a lift tag or pusher axle have the drive axle spec'd for 23K. To accommodate that 20K on a single axle and handle the torque. Not all do, but most are that way. Need to take a look at the axle plate to see the model number.