Scania 420 vs 360

Here's a video that might help you, a well respected mechanic talks about what I mentioned about driving with big weights.
He talks about a Cummins ISX, but the physics applies to all, or most engines.

 

no way.
is there possibility to use single tires on trailer? in some countries law require twin tires

 

yes 14speed would be way better.
in that case stay away from 114 360, 8+c is not even close to what you need.

 

back than it was 16.38 , overdrive was introduced in 2003

 

if we are talking about engine from 3series 360hp. their sweet spot is at 1500rpm for euro spec 40t.
problem with this engine it is low torque and need relatively high rpm for driving, their green field is from 1200-1800rpm. this engines go well and have healthy hp comparing to other brands from that age. but it works well till 40t, for 56t this is not enough, it should use 3.89 ratio for 315/80r22.5. this is very nervous engine, no other engine can change rpm so fast and thereby not very usable for high weights.
420hp can somehow pull this weight, it is marginal but in finland and sweden this was common option.
you need 3.4 (6x2) or 3.42 (6x4) ratio. on long distances with lot of empty runs 3.27 ratio on 6x2 can be alternative.

 

ok how much weight with single tires?
maybe you can use alternative dimmension, twin 265/70 19.5 provide very good balance between fuel economy/liftime/weight/payload

 

with 40t around 10% lower than 113 360.
it is hard to tell with 56t, way more tires, dont know what kind of topography and how smooth is asphalt do you have.
3.08 would be too fast especially for engine with pdf injection.
what dimension of tires would you use?