Quote Originally Posted by the evil twin View Post
Not the most logical of things is it...

On a full chassis vehicle more often than not the engine isn't the lowest common denominator in the equation and it is the transmission or diffs.
On other vehicles it is the chassis load and unrelated to engine/transmission options

When it is the engine that is the limiting factor it is usually things like the engineers assessment of how much extra load the cooling system can handle or somesuch before the reliability might be compromised.
I always thought engine braking capacity played a reasonable part in it to.
Td42, awesome.
Tb45/48 Yea ok.
Zd30... might as well be an auto....... lol