Which truck to buy? axle specs etc

If you're going to pull over 80k and are willing to obtain the permits for each state you'll drive through, then you'll need the 46's. If you needed it, you'd know. Everyone pulls 40's for the most part. The front should only have 14k on it usually if the back weighed more than the 80, but as I'm sure you are aware, you have some say in the matter by where you put your 5th wheel.

 

Only heavy haul needs anything heavier than 12/40 and even then 40 rears will support alot of weight for a long time

 

typical heavy haul spec around here is a 18/46.

normal 12/40 setups will take quite a bit of abuse though.. I "know of" a few local trucks around here that run in the 40-50 range on normal 40K rears, with no issues..

However, the 46K rears will normally get you a deeper ratio, and full locking rears, which are ALWAYS a nice thing to have...

 

Only 90k ? I wouldn't worry about anything with 12/40's The kind of trucks that anything heavier are in the oilfield, hauling really heavy machinery or pulling trains in Michigan. I know several guys that have over 900k miles on 40 rears with 650-750 hp that have had 60-70k on that deck more times than not that have never been apart. Also having 46 rears doesn't allow you to put 46k on your drives and go across the scales. unless you have permits you are only alloud 34k and I've only ever heard of anybody checking the steer axle on car haulers and dump trucksbecause they need to have more than 12k on that axle and in order to have more weight you need wider tires