What is the difference between 40k and 46k rears?

Added strength. The higher the number, the stronger (and heavier) they are.

 

That one is rated at 40k and one is rated at 46k.

 

The 46k also have more gear oil capacity and probably also have thicker housing walls it was an option when I spec’d mine…..

unless your hauling 100k lbs every load the 40k will do the job. But the more important thing in logging is what’s the ratio of the rears ?

 

Either should work for you. I assume you're not worried about being over 80k gross? If not, you can at times find heavy specd trucks cheaper on the used market, because of their added weight it makes it tough to haul a full load for most folks not hauling heavy.

 

Also realize that everything is heavier on 46,000 lbs rears versus a 40,000. Heavier bearings, axles, gears, housing, et cetera.

Everything is more expensive to repair when the time comes.

 

What size rubber?

I’d guess 24.5 , those would be better off road. But at 84K either will be fine. You won’t be in the 8 mpg crowd but it’s a decent working truck set up…..